Rose wrote:

You know, I have occassionally had the same problem (not with 1000
deliveries though) and it was also with attachments. Thanks for
letting us know how to fix it. Is there any particular reason that
happens?

Timing issues with the scanning process.

Shouldn't the email be scanned, deemed safe and just go on
its merry way but not repeat itself?

Ideally, sure, but in reality it often doesn't work that way.
--
Brian Tillman