I've had our Zimbra server working for quite a few months without a hitch. Over the last month, however, I'm experiencing problems delivering messages to Gmail. After 72 hours, the message fails:

Final-Recipient: rfc822; XXXXXX@gmail.com
Action: failed
Status: 4.0.0
Diagnostic-Code: X-Postfix; delivery temporarily suspended: connect to
gmail.com[]: Connection timed out

Thank you in advance.