This article relates to Exim 4, running in a WHM/cPanel environment under Centos, but may affect other configs too.
You may find instances where a local user tries to send mail to a host that operates greylisting. The messages never gets to the recipient. You see things like this in the exim_mainlog
2011-11-10 15:14:05 1ROWKK-0003I1-Ia <= firstname.lastname@example.org H=something.com (FredBlogs) [184.108.40.206] P=esmtp S=7852 id=!&!AAAAAAAAAAAYAAAAAAAAAEDCVk4NrhRJjsshyvaOnAfCgAAAEAAAAOV7jpjiT51Jm/WbyNPkywIBAAAAAAemail@example.com T="FW: test" for firstname.lastname@example.org 2011-11-10 15:14:06 1ROWKK-0003I1-Ia == email@example.com <firstname.lastname@example.org> R=lookuphost T=remote_smtp defer (-44): SMTP error from remote mail server after RCPT TO:<email@example.com>: host mail.host100.co.uk [220.127.116.11]: 451 Greylisted, please try again in 223 seconds 2011-11-10 15:14:06 1ROWKK-0003I1-Ia ** firstname.lastname@example.org: retry timeout exceeded 2011-11-10 15:14:06 1ROWKK-0003I1-Ia Completed