I suggest you disable windows defender
Unfortunately, sometimes there are false positives. I discussed this problem with SM via ticket and also here and they confirmed it to me. Unfortunately, windows defender sometimes generates false positives. The proposed solution was to repeat the scan again in the event of a positive result from Windows Defender. But to date it doesn't seem to me that we have implemented it. So I would say that using defender makes the system unreliable, and therefore until SM finds a solution it shouldn't be used.
I took the trouble to check all the messages that windows defender reports as viruses and to check them manually and I assure you that false positives are not such a rare event, especially with attachments larger than 1Mb
Here is an excerpt from my ticket
Hey Sabatino,I talked to the developers and they said that the way that defender works is when it scans it can sometimes say hey this might be a virus and marks it as a virus then later on once Microsoft does more scans on it and does its internal stuff then it goes this isn't a virus. So this issue is with just the way that defender works. I'm going to make this ticket into a feature request to add the ability for defender to rescan emails then if it comes back as not a virus it will send it through. Kind Regards
Sabatino Traini
Chief Information Officer
Genial s.r.l.
Martinsicuro - Italy