There are various reasons you might receive an error such as:
Connected to <IP address> but connection died
Connected to <IP address> but connection died. Possible duplicate! (#4.4.2)
·Mails stuck in the retry state
1.The firewall is dropping the connection.
If you use a CISCO PIX/ASA firewall, check ESMTP Inspect and disable this feature. You can identify if this feature is enabled by seeing if the 220 Banner is masked when connecting to server via telnet session as seen below.