Amlink Limited
85 Stonor Road, Hall Green
Birmingham B28 0QP
office (0871) 288-6965* / mobile (07917) 340-500

Frequently Asked Questions

Q. Why am I not receiving e-mail sent to me immediately.
Q. There is a large e-mail in my inbox which is stopping delivery of other e-mail. How can I fix this?
Q. Why can't I send e-mails?
Q. Why are e-mails I am sending not being delivered?

Q. Why am I not receiving e-mail sent to me immediately.

A. There could be a number of reasons why e-mail does not appear in your mail box straightaway. The primary reason is that Amlink Limited operate a grey-listing protocol to stop the vast amounts of Spam e-mails that are sent over the internet every day. This is for your benefit to cut down on the amount of unsolicited e-mail you will receive.

Amlink Limited's Greylisting Settings are:

Minutes to defer delivery attempts - 2 Minutes
Days before removing unused records - 1 day (Note 1)
Days before removing used records - 30 days (Note 2)

Note 1: If an email which has been deferred isn't resent within 24 hours the record will be deleted and any retry will be treated as a new e-mail and deferred again.

Note 2: If a sender sends at least one e-mail every 30 days their e-mails wsould never be deferred.
  Greylisting is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate the originating server will, after a delay, try again and, if sufficient time has elapsed, the email will be accepted.

A server employing greylisting deliberately degrades mail service for unknown or suspect sources, over a short period of time. Typically, it records three pieces of data, known as a "triplet", for each incoming mail message:

  • The IP address of the connecting host
  • The envelope sender address
  • The envelope recipient address(es), or just the first of them.

This data is registered on the mail server's internal database, along with the time-stamp of its first appearance. The email message will be dismissed with a temporary error until the configured period of time is elapsed, usually some minutes or a small number of hours. Temporary errors are defined in the Simple Mail Transfer Protocol (SMTP) as 4xx reply codes: Fully capable SMTP implementations are expected to maintain queues for retrying message transmissions in such cases. When a sender has proven itself able to properly retry delivery, it will be whitelisted for a longer period of time, so that future delivery attempts will be unimpeded. For example, a greylister can require a successful delivery attempt against a registered triplet to be no earlier than 25 minutes after registration and not later than 4 hours after it. Repeated delivery attempts before the 25 minute period will be ignored with the same 4xx reply code. After 4 hours the triplet will be expired, so delivery attempts will register anew. When the greylister sees an attempt within the 25 minute - 4 hour window, the connecting host will be whitelisted for 36 days.

The temporary rejection can be issued at different stages of the SMTP dialogue, allowing an implementation to store more or less data about the incoming message. The trade-off is more work and bandwidth for more exact matching of retries with original messages. Rejecting a message after its content has been received allows the server to store a choice of headers and/or a hash of the message body.

In addition to whitelisting good senders, a greylister can provide for exceptions. Greylisting can generally be overridden by a fully validated TLS connection with a matching certificate. Because large senders often have a pool of machines that can send (and resend) email, IP addresses that have the most-significant 24 bits (/24) the same are treated as equivalent, or in some cases SPF records are used to determine the sending pool. Similarly, some e-mail systems use unique per-message return-paths, for example variable envelope return path (VERP) for mailing lists, Sender Rewriting Scheme for forwarded e-mail, Bounce Address Tag Validation for backscatter protection, etc. If an exact match on the sender address is required, every e-mail from such systems will be delayed. Some greylisting systems try to avoid this delay by eliminating the variable parts of the VERP by using only the sender domain and the beginning of the local-part of the sender address.

Why it works

Greylisting is effective because many mass email tools used by spammers do not queue and reattempt mail delivery as is normal for a regular Mail Transport Agent. They do not queue and resend email as this requires the expenditure of resources (spamming normally operates on very narrow margins). Since the advent of greylisting spammers have taken to rerunning their mail delivery tools to resend the same email again without having to expend resources queueing the email. This approach still requires them to expend additional resources resending the email the second time though. As of 2011 spammers have been using this technique for many years.

Delaying delivery also gives real-time blackhole lists and similar lists time to identify and flag the spam source. Thus, these subsequent attempts are more likely to be detected as spam by other mechanisms than they were before the greylisting delay.

Advantages

The main advantage from the users' point of view is that greylisting requires no additional configuration from their end. If the server utilizing greylisting is configured appropriately, the end user will only notice a delay on the first message from a given sender, so long as the sending email server is identified as belonging to the same whitelisted group as earlier messages. If mail from the same sender is repeatedly greylisted it may be worth contacting the mail system administrator with detailed headers of delayed mail.

From a mail administrator's point of view the benefit is twofold. Greylisting takes minimal configuration to get up and running with occasional modifications of any local whitelists. The second benefit is that rejecting email with a temporary 451 error (actual error code is implementation dependent) is very cheap in system resources. Most spam filtering tools are very intensive users of CPU and memory. By stopping spam before it hits filtering processes, far fewer system resources are used. This allows more layers of spam filtering or higher throughput since greylisting can easily be configured as a first line of defense with a heuristic filter such as SpamAssassin handling messages that go through.

Greylisting is particularly effective in many cases at weeding out misconfigured MTAs, and is gaining in popularity as a very effective anti-spam tool. It is likely that those MTAs that do not correctly handle greylisting will become less numerous as greylisting spreads.

Some greylisting packages support a SQL backend which allows for a distributed multiple-server frontend to be deployed with the same greylisting data on all frontends.

Disadvantages

Delayed delivery and its consequences

The biggest disadvantage of greylisting is that for unrecognized servers, it destroys the near-instantaneous nature of email that users have come to expect. Mail from unrecognized servers is typically delayed by about 15 minutes, and could be delayed up to a few days. A customer of a greylisting ISP can not always rely on getting every email in a pre-determined amount of time. This disadvantage is mitigated by the fact that near instantaneous mail delivery is restored once a server has been recognized and is generally maintained automatically so long as users continue to exchange messages. However, this disadvantage is especially visible when a user of greylisting mailserver attempts to reset his credentials to a website that uses email confirmation of password resets. In extreme cases the delivery delay imposed by the greylister can exceed the expiry time of the password reset token delivered in email. In these cases manual intervention may be required to whitelist the websites mailserver so the email containing the reset token can be used before it expires.

Sendmail, one of (if not the most) prolific internet message transport agent has a default retry interval of 15 minutes. Generally this is the maximum amount of time an email will be delayed. Experienced system admins for email systems should tune their mail system settings to sensible values, and the biggest delays from greylisting systems are incurred when communicating with poorly configured sending systems with retry intervals left set at several hours or more.

The original specification for email states that it is not a guaranteed delivery mechanism and not an instantaneous delivery mechanism. This means that greylisting is a perfectly legitimate process and does not break any protocols or rules. Explaining this to users that have become accustomed to immediate email delivery will probably not convince them that a mail server that uses greylisting is behaving correctly.

Modern greylisting applications (such as Postgrey) automatically whitelist senders that prove themselves capable of recovering from temporary errors. Note that this is irrespective of the reputed spamminess of the sender.

When a mail server is greylisted, the duration of time between the initial delay and the re-transmission is variable. Some mail servers use a default of four hours, though most will retry sooner. Most open-source MTAs have retry rules set to attempt delivery after around fifteen minutes (Sendmail default is 0, 15, ..., Exim default is 0, 15, ..., Postfix default is 0, 16.6, ..., Qmail default is 0, 6:40, 26:40, ..., Courier default is 0, 5, 10, 15, 30, 35, 40, 70, 75, 80,..., Microsoft Exchange defaults to 0, 1, 2, 22, 42, 62 ..., Message Systems Momentum defaults to 0, 20, 60, 100, 180, ...). Indeed, SMTP says the retry interval should be at least 30 minutes, while the give-up time needs to be at least 4–5 days.

Greylisting delays much of the mail from non-whitelisted mail servers —not just spam— until typical patterns of communication are recorded by the greylisting system. For best results, whitelisting should be used extensively. A static list of public servers worth being whitelisted can be found in the greylisting.org repository, though this is significantly out-of-date.

Greylisting can be a particular nuisance with websites that require an account to be created and the email address confirmed before they can be used. If the sending MTA of the site is poorly configured, greylisting may delay the initial email containing the signup confirmation link, thus introducing a waiting period even though the actual website may have attempted to send out the email confirmation code immediately. Almost all stock-configured Sendmail MTAs (sendmail being the most widely deployed MTA on the internet) will retry after a few minutes, leading to typical delays of under 10 minutes in most cases (still dependent on the greylisting configuration).

Other problems

On a technical level, some misbehaving SMTP senders may interpret the temporary rejection as a permanent failure. Old clients conforming only to the obsolete specification (RFC 821) and ignoring its recommendations may give up on delivery after the first failed attempt: RFC 821 states that clients "should" retry messages rather than using the word "must". RFC 2119 dictates that "should" means recommended and to ignore at your own risk, and it is a violation of the current SMTP standard for the client to fail to retry. The current SMTP specification (RFC 5321) clearly states that "the SMTP client retains responsibility for delivery of that message" (section 4.2.5) and "mail that cannot be transmitted immediately MUST be queued and periodically retried by the sender." (section 4.5.4.1).

This problem can affect SMTP clients in unexpected ways. Most MTAs will queue and retry messages, but a small number do not. A similar concern exists for applications which act as SMTP clients and fail to incorporate any form of queueing for deferred SMTP mail. This can be mitigated on the sending side by configuring the application to use a local SMTP server as an outbound queue, instead of attempting direct delivery. For the server operator who uses greylisting, clients which are known to fail on temporary errors can be supported by whitelisting or exception lists.

Some MTAs, upon encountering the temporary failure message from a greylisting server on the first attempt, will send a warning message back to the original sender of the message. The warning message is not a bounce message, but it is often formatted similarly to one and reads like one. This practice often causes the sender to believe that the message has not been delivered, when in fact the message will be delivered successfully at a later time.

Also, legitimate mail might not get delivered if the retry comes from a different IP address than the original attempt. When the source of an email is a server farm or goes out through some other kind of relay service, it is likely that a server other than the original one will make the next attempt. For network fault tolerance, their IPs can belong to completely unrelated address blocks, thereby defying the simple technique of identifying the most significant part of the address. Since the IP addresses will be different, the recipient's server will fail to recognize that a series of attempts are related, and refuse each of them in turn. This can continue until the message ages out of the queue if the number of servers is large enough. This problem can partially be bypassed by proactively identifying as exceptions such server farms. Likewise, exception have to be configured for multihomed hosts and hosts using DHCP. In the extreme case, a sender could (legitimately) use a different IPv6 address for each outbound SMTP connection.

A sender subjected to greylisting might move to a backup server and reattempt delivery. In order for greylisting to work in such cases, all backup mail servers (as specified by lower-priority MX records for the domain) should implement the same greylisting policy and share the same database. Traffic to those backup servers increases merely as a result of greylisting.

Other reasons you may not receive e-mail:

The senders mail server may refuse to send the e-mail after it has checked various databases. Use the resources page to check senderbase.org for sender reputation. Our IP address is 159.253.211.96. If the reputation is not neutral or good please contact us immediately. You may also wish to check if the IP address has been blacklisted.

We make checks on a regular basis and try to ensure that the businesses using our mail server do not send spam however, now and again an e-mail account may become compromised and will be able to send spam. As soon as this comes to our attention we do take steps to stop the spam.

Q. There is a large e-mail in my inbox which is stopping delivery of other e-mail. How can I fix this?
A. Although large e-mails can be sent and received, it is much better to send several smaller e-mails. When generally advise that no e-mail should exceed 100MB in size. Many e-mail clients will timeout before the e-mail is received and start downloading repeatedly. To delete a very large e-mail which is stuck log into your e-mail account using the webmail which can be accessed through http://webmail.yourdomainname.co.uk (or .com etc.). Then request the sender to send several smaller e-mails.

Q. Why can't I send e-mails?
A. This is normally because you have not authenticated as a user of our services.

In Outlook go to Tools and then Account Settings, click on the e-mail box and select Change. Click on More Settings and select the Outgoing Server Tab. A tick is required in the box "My outgoing server (SMTP) requires authentication". You should use the same settings as are used for the incoming mail server.
Q. Why are e-mails I am sending not being delivered?
A. Firstly check that you have spelt the recipients e-mail address correctly. This is teh most common reason for e-mail not being delivered.

There are a number of other reasons that your e-mail may not be delivered. If you have received a bounce back report. Have a look for the SMTP error code which should give you the reason the e-mail has been rejected:

CODE MEANING HOW TO SOLVE IT/WHAT TO DO
420 "Timeout connection problem": there have been issues during the message transfer

This error message is produced only by GroupWise servers. Either your email has been blocked by the recipient's firewall, or there's a hardware problem. Check with your provider.

421

The service is unavailable due to a connection problem: it may refer to an exceeded limit of simultaneous connections, or a more general temporary problem.

The server (yours or the recipient's) is not available at the moment, so the dispatch will be tried again later.

422

The recipient's mailbox has exceeded its storage limit.

Best is to contact contact the user via another channel to alert him and ask to create some free room in his mailbox.

431

Not enough space on the disk, or an "out of memory" condition due to a file overload.

This error may depend on too many messages sent to a particular domain. You should try again sending smaller sets of emails instead of one big mail-out.

432

Typical side-message: "The recipient's Exchange Server incoming mail queue has been stopped".

It's a Microsoft Exchange Server's SMTP error code. You should contact it to get more information: generally it's due to a connection problem
441 The recipient's server is not responding

There's an issue with the user's incoming server: yours will try again to contact it.

442 The connection was dropped during the transmission

A typical network connection problem, probably due to your router: check it immediately.

446

The maximum hop count was exceeded for the message: an internal loop has occurred.

Ask your SMTP provider to verify what has happened.

447 Your outgoing message timed out because of issues concerning the incoming server

This happens generally when you exceeded your server's limit of number of recipients for a message. Try to send it again segmenting the list in different parts.

449

A routing error.

Like error 432, it's related only to Microsoft Exchange
450

"Requested action not taken – The user's mailbox is unavailable". The mailbox has been corrupted or placed on an offline server, or your email hasn't been accepted for IP problems or blacklisting.

The server will retry to mail the message again, after some time. Anyway, verify that is working on a reliable IP address.

451

"Requested action aborted – Local error in processing". Your ISP's server or the server that got a first relay from yours has encountered a connection problem.

It's normally a transient error due to a message overload, but it can refer also to a rejection due to a remote antispam filter. If it keeps repeating, ask your SMTP provider to check the situation. (If you're sending a large bulk email with a free one that can be a common issue).

452

Too many emails sent or too many recipients: more in general, a server storage limit exceeded.

Again, the typical cause is a message overload. Usually the next try will succeed: in case of problems on your server it will come with a side-message like "Out of memory".

471

An error of your mail server, often due to an issue of the local anti-spam filter.

Contact your SMTP service provider to fix the situation.

500 A syntax error: the server couldn't recognize the command

It may be caused by a bad interaction of the server with your firewall or antivirus. Read carefully their instructions to solve it.

501

Another syntax error, not in the command but in its parameters or arguments.

In the majority of the times it's due to an invalid email address, but it can also be associated with connection problems (and again, an issue concerning your antivirus settings).

502

The command is not implemented.

The command has not been activated yet on your own server. Contact your provider to know more about it.

503

The server has encountered a bad sequence of commands, or it requires an authentication.

In case of "bad sequence", the server has pulled off its commands in a wrong order, usually because of a broken connection. If an authentication is needed, you should enter your username and password.

504

A command parameter is not implemented.

Like error 501, is a syntax problem; you should ask your provider.

510
511

Bad email address.

One of the addresses in your TO, CC or BBC line doesn't exist. Check again your recipients' accounts and correct any possible misspelling.

512

A DNS error: the host server for the recipient's domain name cannot be found.

Check again all your recipients' addresses: there will likely be an error in a domain name
513

"Address type is incorrect": another problem concerning address misspelling. In few cases, however, it's related to an authentication issue.

Doublecheck your recipients' addresses and correct any mistake. If everything's ok and the error persists, then it's caused by a configuration issue (simply, the server needs an authentication).

523

The total size of your mailing exceeds the recipient server's limits.

Re-send your message splitting the list in smaller subsets.

530

Normally, an authentication problem. But sometimes it's about the recipient's server blacklisting yours, or an invalid email address.

Configure your settings providing a username+password authentication. If the error persists, check all your recipients' addresses and if you've been blacklisted.

541

The recipient address rejected your message: normally, it's an error caused by an anti-spam filter.

Your message has been detected and labeled as spam. You must ask the recipient to whitelist you.

550

It usually defines a non-existent email address on the remote side.

Though it can be returned also by the recipient's firewall (or when the incoming server is down), the great majority of errors 550 simply tell that the recipient email address doesn't exist. You should contact the recipient otherwise and get the right address.

551

"User not local or invalid address – Relay denied". Meaning, if both your address and the recipient's are not locally hosted by the server, a relay can be interrupted.

It's a (not very clever) strategy to prevent spamming. You should contact your ISP and ask them to allow you as a certified sender.
552

"Requested mail actions aborted – Exceeded storage allocation": simply put, the recipient's mailbox has exceeded its limits.

Try to send a lighter message: that usually happens when you dispatch emails with big attachments, so check them first.

553

"Requested action not taken – Mailbox name invalid". That is, there's an incorrect email address into the recipients line.

Check all the addresses in the TO, CC and BCC field. There should be an error or a misspelling somewhere
554

This means that the transaction has failed. It's a permanent error and the server will not try to send the message again.

The incoming server thinks that your email is spam, or your IP has been blacklisted.