We are having problems receiving email from gmail.com accounts. I have found that we only receive emails from gmail accounts that have been formatted in rich text. If a message is sent from the same account in plain text, I receive the following returned message after several days:

Connected to external_IP_address_of_mail_ server but connection died. Possible duplicate! (#4.4.2)

Has anyone heard of anything like this before? Is the problem not with the mail server but perhaps the firewall?