Mail to AOL addresses bouncing.

One of my clients is having an issue with mail form his domain to any address at AOL.com being rejected with the following error...

Code:

This message was created automatically by mail delivery software.
A message that you sent has not yet been delivered to one or more of its
recipients after more than 24 hours on the queue on hostname.mydomain.com.

The message identifier is:     1cV64Y-0006bB-M8
The subject of the message is: Download Links
The date of the message is:    Sat, 21 Jan 2017 19:31:54 -0500

The address to which the message has not yet been delivered is:

 trecipientname@aol.com
   host mailin-02.mx.aol.com [64.12.88.163]
   Delay reason: SMTP error from remote mail server after initial connection:
   421 mtaig-mba02.mx.aol.com Service unavailable - try again later

No action is required on your part. Delivery attempts will continue for
some time, and this warning may be repeated at intervals if the message
remains undelivered. Eventually the mail delivery software will give up,
and when that happens, the message will be returned to you.
Reporting-MTA: dns; hostname.mydomain.com

I read an old post about this and I THINK everything is set correctly. DKIM and SPF seem correct. Any ideas on how to resolve this issue?

 

I used Reverse DNS check and the IP being used does properly resolve to
hostname.mydomain.com

 

I went to that site and it made absolutely zero sense to me. It looks like the problem MIGHT be that the MTA is hosname.myserverdomain.com and the client is sending mail form his account on user@hisdomain.com, thus the domain names do not match. WOudl this cause the problem and if that si the cause, how do I address it?