I found a Plesk 8.3 server running RHEL 4 last month that was presenting errors when users attempted to send e-mail via SquirrelMail:
ERROR:
Email delivery error
Server replied: 127 Can’t execute command ‘/usr/sbin/sendmail -i -t -fsomeuser@somedomain.com’.
The error was appearing because safe_mode was enabled and SquirrelMail was unable to drop e-mails into /usr/sbin/squirrelmail. After disabling safe_mode on [...]
I’ve used this extremely basic procmail configuration a million times, and it’s a great start for any server configuration. It passes e-mails through spamassassin (if they’re smaller than 256KB) and then filters any e-mail marked as spam to /dev/null:
LOGFILE=/var/log/procmail.log
DROPPRIVS=yes
:0fw
| /usr/bin/spamc
:0
* ^X-Spam-Status: Yes
/dev/null
Of course, you can make this much more complicated with some additional customization.
If you see a large mail queue and your system’s I/O is increasing, you may find messages like these in your syslog:
Losing q5/qfg9N5EwE3004499: savemail panic
SYSERR(root): savemail: cannot save rejected email anywhere
In this situation, there’s some reason why sendmail cannot deliver e-mail to the postmaster address. There’s a few issues that can create this problem:
Missing [...]
If you’ve ever worked on a server that was being overworked (I know I have), you might see sendmail throw out a message like this one:
Jan 9 14:15:49 mail sendmail[18184]: rejecting connections on daemon MTA: load average: 70
This is sendmail’s way of trying not to contribute to the already high load placed on the server. [...]