Department of Engineering

Automated filters on incoming email

Introduction

Electronic mail increasingly presents two problems for users: unsolicited bulk email ('spam'); and computer viruses carried as attachments to email messages. To minimise both of these problems, email to which is delivered via the Engineering Department servers is subject to automated filters intended to reject suspect messages. This page outlines the filtering on mail coming into eng.cam.ac.uk. It also advises on what you can do if you believe that legitimate email intended for you is being rejected.

All personal email is now delivered to Hermes, but if it is sent via an @eng.cam.ac.uk address it will still be subject to the Spam filters below, although we do not do any virus scanning of this mail. Such email will also be subject to the restrictions and filters on Hermes. See the Computing Service documentation on Spam and other unwanted mail and Mail viruses.

'Spam' filters

These filters are intended to block unsolicited bulk email, usually unwanted advertising material. The department uses a number of block lists to reject mail. They generally consist of a database of addresses that have been identified as generating, or allowing the transmission of, unwanted email. The lists we're currently using are the SBL and the XBL, but these may change from time to time.

In addition, the Department's system also blocks mail coming from sources which have previously tried to send junk or abusive email to the Department.

Despite these blocks, some unwanted mail may still get through. Our page on junk email advises on what you can do if you receive such mail, and how to report it.

What to do if the filters are blocking genuine email or attachments

If you find that legitimate attachments intended for you are being blocked by the filters, it should be possible to find a way round the block. The best method is to ask the sender to enclose the attachment in a zip file (the standard PC means of archiving and compressing files). If this is not possible, ask them to rename the attachment with a non-standard filename extension (e.g. remove the '.exe'). When you receive it, rename it back to the original name before opening it. If you require further help, please contact helpdesk for advice.

If the blocked message did not contain an attachment, or was returned to the sender with an error message starting "Access denied", the message has probably been blocked by one of the 'spam' filters. In this case there are four possibilities:

  • If the sender uses a dial-up Internet service or a dynamically allocated port, it is possible that they are not submitting mail via their ISP's official outbound mail server. (See http://mail-abuse.org/dul/enduser.htm for why this is blocked.) They can easily solve the problem by setting their mail program to use their ISP's official mail server.
  • The mail has been blocked because the source is in the SBL list of suspect addresses. In this case the error message should contain a URL which gives more details as to why the source is blocked. In general sites in SBL which are no longer sources of junk email should follow the information found at http://www.spamhaus.org/ to get themselves removed from SBL.
  • Otherwise, the message is probably coming from a site or address range that we have identified as a source of junk or abusive mail in the past. In this case, the best course of action is to ask the sender to forward a copy of the complete rejection message to postmaster (mail to postmaster will not be rejected). Postmaster can then investigate and try to find a solution.

If all else fails email postmaster with as many details as possible about the sender of the mail (e.g. email address, site sending the mail, and when it was failing).

The Limitations of Virus Filtering

Email has become one of the most common ways in which computer viruses are spread: a virus is sent as an email attachment, disguised as something harmless and often appearing to come from an acquaintance. When the attachment is opened, the virus infects the computer and spreads itself further.

What the attachment filters do not do
The filters on email attachments do not mean that you can do without anti-virus software on your computer. They merely reduce the chances of a virus program getting through to you and fooling you into opening it. You must still take care when handling email attachments, and open them only if you know both who they are from and that they are safe. Make sure that you have up-to-date anti-virus software on your computer. Information on anti-virus software is available from the Computing Service.

If you ever receive a message and are worried that it may be a virus you can always email the postmaster team on postmaster-query and we will check it over for you.