getting user unknown error on AOL accounts

I have setup Zoho to be a mail hosting system.

When I send mail to the two accounts setup so far, I can send mail from several other mail accounts and they go through fine.

But two people with AOL accounts are getting bounce back error messages and their mail is not coming through:

This is the error message they receive:

----- The following addresses had permanent fatal errors -----
<info@seniorpremierdating.com>
(reason: 550 5.1.1 <info@seniorpremierdating.com>... User unknown)

----- Transcript of session follows -----
... while talking to mx.zohomail.com.:
>>> RCPT To:<info@seniorpremierdating.com>
<<< 550 5.1.1 <info@seniorpremierdating.com>... User unknown
550 5.1.1 <info@seniorpremierdating.com>... User unknown

Original-Recipient: rfc822;info@seniorpremierdating.com
Final-Recipient: RFC822; info@seniorpremierdating.com
Action: failed
Status: 5.1.1
Remote-MTA: DNS; mx.zohomail.com
Diagnostic-Code: SMTP; 550 5.1.1 <info@seniorpremierdating.com>... User unknown
Last-Attempt-Date: Sun, 3 Jun 2012 13:43:12 -0400

So far this only seems to be happening with AOL accounts - but we haven't received a lot of mail yet.

Rick