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.

Restart sendmail: $ 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.