Hi. I have been experiencing a similar problem. Recently, I know of 3 emails I never received. The error message given to 1 sender was:

This Message was undeliverable due to the following reason:

Your message was not delivered because the destination computer was
not reachable within the allowed queue period.  The amount of time
a message is queued before it is returned depends on local configura-
tion parameters.

Most likely there is a network problem that prevented delivery, but
it is also possible that the computer is turned off, or does not
have a mail system running right now.

Your message was not delivered within 4 days and 0 hours.
Host cdptpa-ow2ms-mta-svip-01.email.rr.com is not responding.

The following recipients did not receive this message:

     (removed fro security reasons)

Please reply to <*** Email address is removed for privacy ***>
if you feel this message to be in error.

Original-Recipient: RFC822;(removed fro security reasons)
Final-Recipient: RFC822; <(removed fro security reasons)
Action: failed
Status: 4.4.7
Remote-MTA: dns; cdptpa-ow2ms-mta-svip-01.email.rr.com
X-Actual-Recipient: RFC822; (removed fro security reasons)

Please let me know if anyone can help!