I had the same problem. The solution to activate SMTP Auth on Microsoft Entra is unsafe and we don’t know how long this will last.
I wrote instead a relay that turns the SMTP Auth Emails into Graph Compatible form and sends it safe. It runs in the same Docker stack as my ERP Next. I do recommend this solution. Just fork my repo, host the image yourself and implement it next to your ERP Next container.
The README.md describes it all pretty well. Please leave a star at my Repo and report upcoming issues, so I can help solve them.