sendmail, 4th Edition

m=

Max messages per connection V8.10 and later

The m= delivery agent equate is used to limit the number of envelopes that can be delivered during any single SMTP or LMTP connection.^[279] This can prove useful because more and more sites on the Internet have started rejecting envelopes after too many have been sent.

For example, consider a malicious user at your site who wants to advertise to thousands of users at aol.com by sending a single envelope to all of them. By defining this m= delivery agent equate to a value of, say, 25, only the first 25 envelopes would be delivered on the initial connection to aol.com. For the 26th, sendmail would have to reestablish the connection to send the next 25. Thousands of envelopes would require a new connection for each group of 25 envelopes, thus slowing the flow and giving you more time to detect the affront.

The way to add this delivery agent equate to your SMTP delivery agents looks like this:

define(`SMTP_MAILER_MAXMSGS', `25')

This m= delivery agent equate can also be used with FEATURE(local_lmtp) (FEATURE(local_lmtp) on page 625), which causes mail.local to accept envelopes via LMTP. This can be useful if your machine receives many envelopes from another machine for local delivery. Instead of expecting mail.local to deliver hundreds of envelopes locally during a single run, you can reduce the stress on your machine by limiting the number of envelopes to a comfortable few. Consider defining the following in your .mc configuration file:

define(`LOCAL_MAILER_MAXMSGS', ...