Users gets sporadic System Undeliverable messages from good email addresses
I'm getting backed into a corner with a user we have that is running into more and more problems with sending emails outside our company organization. This seems to be completely sporadic. It is only happening on external contacts. Hes tried to directly type the email address in, click on mailto links, find them in directories, and nothing seems to make a difference. The message that almost instantly comes back is the following:
Your message did not reach some or all of the intended recipients.
Subject: Meeting at Company
Sent: 9/26/2012 9:02 AM
The following recipient(s) cannot be reached:
'email@example.com' on 9/26/2012 9:02 AM
None of your e-mail accounts could send to this recipient.
Why does this keep happening, and what do i do to correct it?