undetermined problem with outgoing mail

I have a customer who is sporadically unable to send mail. Sometimes it happens sometimes it doesn't. I've searched just about everything on this board regarding SMTP and tried some of the fixes. Checked the DNS and everything I know to check.

I'm using cpanel/WHM and I've tried updating and restarting Exim. The customer is having problems on both Outlook for Mac OS9 and Mail for Mac OSX, two different email accounts, same domain/hosting account. In outlook they get a message server not found. In Mail the email goes to the outbox and stays there. Other customers on the same server have no problems, sometimes they are even able to send mail.

Normally, I'd suspect that their settings in their email clients are wrong, but I've actually had this problem before as well so I'm pretty confident there is actually some sort of problem.

Is it possible there's some sort of conflict with our ISP's? What are some things I can try? Any help pinpointing or fixing this problem is appreciated.

 

 

 

 

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