Often fails to send mail under ASA

After replacing ASA5525, it was recently discovered that some people often failed to send individual emails. The code 0x800 CCC 0F appeared on Outlook.

Check the network and there is no problem. It is found that this email was dropped when passing ASA5525 through packet capture. Check the log on ASA and find that the packet loss code on the log is:

Aug 03 2018 09:36:16: %ASA-6-305011: Built dynamic TCP translation from inside:10.10.200.148/52065 to outside:118.242.17.18/52065

Aug 03 2018 09:36:16: %ASA-6-302013: Built outbound TCP connection 181126021 for outside:58.215.117.69/25 (58.215.117.69/25) to inside:10.10.200.148/52065 (118.242.17.18/52065)

Aug 03 2018 09:36:17: %ASA-4-108004: ESMTP Classification: Dropped connection for ESMTP Request from inside:10.10.200.148/52065 to outside:58.215.117.69/25; matched Class 4: header line length gt 998

So what is the reason for "%ASA-4-108004: ESTP Classification: Dropped connection for ESTP Request from inside: 10.10.200.148/52065 to outside:58.215.117.69/25; matched Class 4: header line length gt 998"?

Looking for Cisco solutions, I found similar people asking the same question.

The solution given by God is that the firewall chooses packet loss when checking esmtp

So I decided to remove the policy check for esmtp (which was the default policy)

After deleting it, I grabbed the package again. Sure enough, there was no problem that the mail could not be sent again.