Core issue
This issue occurs when the Simple Mail Transfer Protocol (SMTP) inspection is enabled in the firewall.
When this issue occurs, the %ASA-6-302014: Teardown TCP connection **** for outside: xxxx to inside:xxxx duration X:X:X bytes xxxx Flow closed by inspection error message appears.
This issue appears due to the presence of Cisco bug ID CSCsh33982.
The main cause of this issue is that a body packet drops from an SMTP communication, and the client retransmits until it stops.
In the syslogs, there is a teardown due to the inspection.
Resolution
For a temporary workaround:
Use the no inspect esmtp command in order to disable the SMTP inspection.
In order completely resolve the issue:
Upgrade to the ASA version 7.2(2.12) or later. In order to download latest software, refer to Cisco Downloads.
