I could never get SMTPSecuresocket to reliably work with office365. It would work intermittently but under load it would often fail. So I switched to using Monkeybreads CURLEmailMBS. It has worked without fail since changing to it.
Here I have a writeup on converting my existing code to use CURLEmailMBS:
https://forum.xojo.com/t/replacing-smtpsecuresocket-mbs-need-tls-1-3/70367