The error you see in the logs related to ZIP files is not related to the repeated emails. I say that because filtering in Xeams occurs in a different thread. The primary thread, which accepts the email is only responsible for downloading the message.

This is the reason why the time between the start of the email and the end is less than 3 seconds. See below.

2021-03-13 10:35:11,089 - [ 4048] S <-- 354 Start mail input; end with <CRLF>.<CRLF>
2021-03-13 10:35:14,884 - [ 4048] S <-- 250 2.6.0 Queued mail for delivery

For some reason, the sender is terminating the connection before sending a QUIT command. You will have to contact the sender and see their logs to determine why does it keep sending the message over and over again.

Now, let me talk about XLS files. Xeams tries to look inside Excel sheets to see if they have harmful contents. Files with an extension *.XLSX are actually zipped. Often folks rename *.XLS files to *.XLSX to avoid detection. You would see an error logged when this happens. Rest assured, this error has nothing to do with the repeated emails you're getting.