IMAP/SMTP failing for one account

I have a client who is unable to send mail. This client is the only one reporting this problem.

This is a cPanel box + RHEL3
Exim as the MTA.

Here's the error:
Server: 'hostname', Protocol: SMTP,
Server Response: '421 Unexpected failure, please try later',
Port: 25, Secure(SSL): No,
Server Error: 421, Error Number: 0x800CCC79

Server: 'hostname', Protocol: SMTP,
Server Response: '421 Unexpected failure, please try later',
Port: 26, Secure(SSL): No,
Server Error: 421, Error Number: 0x800CCC79

Server: 'mail.domain.com', Protocol: SMTP,
Server Response: '421 Unexpected failure, please try later',
Port: 25, Secure(SSL): No,
Server Error: 421, Error Number: 0x800CCC79

Server: 'mail.domain.com', Protocol: SMTP,
Server Response: '421 Unexpected failure, please try later',
Port: 26, Secure(SSL): No,
Server Error: 421, Error Number: 0x800CCC79
As you can see - I had the client try both port 25 and port 26. As well as their domain and the hostname as the SMTP host.

0x800CCC79 = mail server rejected recipients. Why I don't know - waiting on the client to provide an example of the recipeients. Though if all mail is not sending - I'm thinking it's something else.....

421 Unexpected failure, please try later = the probable cause. Anyone know what this might be? I googled a little bit - but I didn't find much. Or I just don't know how to google well....

Thank you!

 

 

 

 

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