I get lots of these from AOL:
    **********************************************
    **      THIS IS A WARNING MESSAGE ONLY      **
    **  YOU DO NOT NEED TO RESEND YOUR MESSAGE  **
    **********************************************
The original message was received at Sun, 11 Dec 2005 03:55:35 -0500
from localhost.localdomain [127.0.0.1]
   ----- Transcript of session follows -----
... while talking to mailin-04.mx.aol.com.:
>>> DATA
<<< 421-:  (DYN:T1)  
http://postmaster.info.aol.com/errors/421dynt1.html<<< 421 SERVICE NOT AVAILABLE
... while talking to mailin-02.mx.aol.com.:
>>> DATA
<<< 421-:  (DYN:T1)  
http://postmaster.info.aol.com/errors/421dynt1.html<<< 421 SERVICE NOT AVAILABLE
... while talking to mailin-01.mx.aol.com.:
>>> DATA
<<< 421-:  (DYN:T1)  
http://postmaster.info.aol.com/errors/421dynt1.html<<< 421 SERVICE NOT AVAILABLE
... while talking to mailin-03.mx.aol.com.:
>>> DATA
<<< 421-:  (DYN:T1)  
http://postmaster.info.aol.com/errors/421dynt1.html<<< 421 SERVICE NOT AVAILABLE
<"snipped"@aol.com>... Deferred: 421-:  (DYN:T1)  
http://postmaster.info.aol.com/errors/421dynt1.htmlWarning: message still undelivered after 4 hours
Will keep trying until message is 5 days old
The people at AOL have confirmed this to be because of improper headings:
X-ClientAddr: 127.0.0.1
Return-Path: <apache@secure.blueplatehost.com>
It's the user apache in the address above that gets the confirmation email blocked. Return-Path for mailings are set to bounce@xxx.com so there's no roblem there. 
How can I correct this for confirm & welcome emails?