Anti-spam policy

The Mail Transport Agents (MTAs) for are configured to reject some mail in an effort to reduce spam and other mail problems. If your mail is rejected by this policy, you will receive a response directing you to this page.

The criteria under which a message may be rejected are:

DNS Block Lists
The connecting host is listed in one of the DNS block lists we use.
Standards violation
The message transport attempt violated the internet standards for mail transport.
Bad attachment
The message contains a dangerous or unwanted file attachment.
Message size
The message is too large.

DNS Block Lists

DNS Block Lists (DNSBLs) are used to perform a real-time lookup of the IP address of any host attempting to send mail to this domain. If the connecting host's address is listed as a misbehaving system, mail from that host is rejected.

If your mail is rejected by this, it means your mail service provider is listed as a "bad citizen" on the Internet. This may be due to malice or mere carelessness on the part of the administrator of the machine.

If you are reading this page, your mail was not blocked because you are necessarily a bad person. (Originators of malicious mail will almost certainly never read this page, because they don't care enough to find out why their mail is not delivered.) What it does mean is that your mail service provider needs to stop their system doing whatever misbehaviour caused them to be listed on these block lists.

These are the DNSBL services used, with links to the pages explaining the listing policy for each:

Standards violation

Failure to follow the internet standards for mail transport is often indicative of misbehaving mail systems, whether through malice or error. A number of checks are made to reject faulty message transport attempts.

Mail systems that are properly configured should not fall foul of these checks; if in doubt, your mail service provider should check the rejection message against the Internet standards documents for mail.

Bad attachment

The email system was designed to transport text messages. Later improvements such as <acronym title="Multipurpose Internet Mail Extensions">MIME</acronym> allow messages to carry content other than plain text, like international writing systems or cryptographic signatures. These features are sometimes used to "attach" files to messages.

While it can sometimes be convenient for people to send a data file or a document attached to a message, it is usually far better to place that file online and inform the recipient of its location. This is better because the recipient then has a choice of the method, timing and location used to retrieve the file (and even whether to retrieve it at all).

Executable code (programs) can be attached just as easily as data files. Unlike data files, though, there is rarely a benefit to sending executable content via email. Its main effect, far from being beneficial, is to allow the spread of email worms and other malicious code. Although these do less damage to correctly-designed mail programs (ones that do not execute the content), they still cause collateral damage as huge files are sent to arbitrary addresses by infected computers.

Messages that contain dangerous or unwanted file attachments will be rejected. Any executable file formats should not be sent in email, and many other formats can cause unwanted behaviour in mail software.

Message size

A legitimate email message is composed mostly of readable plain text; most message "attachments" are dangerous or otherwise unwanted.

Without such unwanted message content, most messages are typically far smaller than a document file. An extraordinarily large message is almost never legitimate. The MTAs for will reject any large message (currently defined as larger than 5MB).