Incoming emails are processed for spam in several different ways. This document is intended to detail the message flow delineating each point where spam processing takes place and the end-user's controls over the filtering process.

Message Flow:

Oxmail Mail Relay

  • Virtually all external email (i.e. from the public internet) to an Oxford address is directed at the Oxmail mail relay cluster.
  • Oxmail performs various SMTP time checks, scans for viruses and applies a score for the likelihood that the message is spam. An ‘X-Oxmail-Spam-Level’ header is applied with ‘*’ symbols corresponding to the score.
  • Messages that pass the virus and address tests are forwarded to Nexus (or to other University email systems if the address isn't handled by Nexus). This happens regardless of the spam level assigned.
  • Messages submitted from Oxford Univeristy hosts to the Oxmail relay are not scanned or spam scored.

Nexus

  • Converts Oxmails' spam score to a numerical value (the 'spam confidence level' or SCL).
  • If the email address or domain have been whitelisted (in Outlook Web Access or Microsoft Outlook), no further scanning is performed and the message is delivered straight to the inbox. N.B. We recommend that users don't whitelist .ox.ac.uk addresses as this could permit spoofed email to be delivered. Genuine ox.ac.uk addresses are unlikely to be scored as spam other than by client-side software.
  • Nexus checks to see if spam filtering is enabled for this user (turned on in Outlook Web Access)
  • The selected level is applied (off, low, medium or high – all set in Selfreg's 'spam email filter settings'). Messages are only moved to the 'junk' folder if they exceed the value that's been set (Low=7, Medium=5, High=3). Email scored as '-1' is defined as 'cannot be spam', usually because it's an internal message. N.B. Outlook users should rely on the servers' filtering, not their client software.

User interaction and control

  • Spam filtering turned on or off: Outlook Web Access
  • Whitelisting: Outlook Web Access or Outlook client
  • Adjust preferences for spam level: Self registration service
  • Further filtering: Email client
Detailed diagram of spam processing
Figure 1. How spam is dealt with by central services