Invitation E-mail is Never Received

I have sent an invitation e-mail to one of my users, but the e-mail is never received. I have been monitoring our mail server log file, and the e-mail has never arrived (after more than 24 hours). Three other people received their invitations without problem. I have deleted this user and re-sent the invitation several times, but the e-mail is never received. It's not being blocked by a spam filter; it's never seen by our mail server. What can I do to either resolve or troubleshoot this further?