I am getting this error for just this user on one of my lists.  When he
tries to manually login to my listserv site he gets this error:
"SonicWALL - Blocked by Application Firewall".  I think his firewall is
blocking my listserv emails because he is the only account on that list
that gets an error message back to me. 
 
This is what I receive from my listserv.
 
Your message did not reach some or all of the intended recipients.
 
      Subject:         Re: Modified Early Warning System
      Sent:               3/11/2010 10:16 AM
 
The following recipient(s) cannot be reached:
 
      [log in to unmask] on 3/11/2010 11:49 AM
            There was a SMTP communication problem with the recipient's
email server.  Please contact your system administrator.
            <mail.AARC.ORG #5.5.0 smtp;550 Invalid sender address
(#5.7.1)>
 
I am just trying to cover all my bases before I tell his IT department
to look at their side.  Has anyone had this error that ended up being
their fault.
 
Thanks.
 
Russell W. Leighton, Jr.
American Association for Respiratory Care
IT Coordinator
9425 N. MacArthur Blvd, Suite 100
Irving, TX  75063
Direct:  972-406-4671
Fax:  972-484-2720