6.3.3 Performance notes
One of the surest performance killers for Sendmail users is when
Sendmail is configured to synchronously verify the recipient's host via
DNS. If it does this for messages posted to it from Mailman, you will
get horrible performance. Since Mailman usually connects via localhost
(i.e. 127.0.0.1) to the SMTP port of Sendmail, you should be sure to
configure Sendmail to not do DNS verification synchronously for
localhost connections.
