I've had an annoying hiccup on a server at work. It's my first production linux server for an outside company for web serving. PHP, Apache, MySQL, and Sendmail are all working great by themselves, but php pages will not use the mail() command. The page goes through immediately, but the mail is never sent.
I tried adding apache as a trusted user in sendmail. I tried making sure that the path to sendmail was correct. Then I tried running a test by typing php mail.php as root from SSH... and it worked.
So I logged into SSH as another user, ran the command again and got
WARNING: RunAsUser for MSP ignored, check group ids (egid=506, want=51)
can not chdir(/var/spool/clientmqueue/): Permission denied
Program mode requires special privileges, e.g., root or TrustedUser.
A simple chmod -R 777 /var/spool/clientmqueue/ cleared up the problem.
So that's my linux hint for the day.