Our Ubuntu web host, hosted with OSG, was not able to send mail (using PHP mail) outside of UF. An OSG tech said our From: header should be a valid address at UF (check) and that the logs at smtp.ufl.edu showed those messages never made it there.
The solution was to configure sendmail to use smtp.ufl.edu as the “smart” relay host (as it’s described in the config file):
$ sudo nano /etc/mail/sendmail.cf
Ctrl+w and search for
smart. On the line below, add
smtp.ufl.edu directly after
DS with no space. The result should be:
# "Smart" relay host (may be null) DSsmtp.ufl.edu
Ctrl-x and save the buffer.
$ sudo /etc/init.d/sendmail restart
As soon as I did this, the queued messages were sent out. I still don’t know why messages to ufl.edu succeeded while others sat in the queue.