When sending emails from Apple mail with attachments or perhaps just Mac an additional attachment ” nameless.htm” gets added as an attachment.

In Microsoft 365 , if you have your malware filter enabled and .htm , .html in the list of attachments to block,  legitimate emails will get blocked.

we resolved the issue as follows.

1. Do not add .htm & html to the list of files in malware filter.

2. Create a transport rule as shown below.

Works like a charm