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How To Tweak Sendmail Queue Priority To Relay Email With Attachment Instantly?

The default Sendmail configuration does not instantly relay emails with big attachments. If you find a good reason to tweak it, then just do it.

With reference to Sendmail tuning guide, each email message instantiated is assigned a priority defined by this formula:
priority = size - (pre x cfactor) + (nrecpt x rfactor) 

  • size is email message size in byte.
  • pre is email precedence / importance class (e.g. high, low, or normal).
  • cfactor is one of configurable weight; default is 1800 (v8.13.8).
  • nrecpt is number of recipient in the email.
  • rfactor is one of the configurable weight; default is 30000 (v8.13.8).

The higher priority value derived from the above formula means the email message will be kept in the queue longer – that’s why emails with big attachments are not relayed or sent immediately!

Therefore, you need to tweak cfactor and/or rfactor if you want to ensure emails with big attachments are sent immediately:
  1. Edit /etc/mail/sendmail.cf (the Sendmail configuration file on RHEL5.2)
  2. Locate O RecipientFactor and O ClassFactor, remove comment character “#” if the options are remarked, and change the default value to bigger one.
  3. Save the sendmail.cf file.
  4. Restart Sendmail daemon for changes to take effective:
    service sendmail restart

NOTE, tweak the Sendmail queue priority carefully, as this might severely impact server and network performance in the case of virus attack!

If an email with big attachment has to be sent soonest possible, ask user to change the email message precedence / importance setting from normal to high.

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