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The mail traffic is unable to pass through the ASA firewall, and 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

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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.

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