You can send emails via Mimecast instead of 365 , so you don’t need a licensed 365 user.
Login and enable SMTP Email submissions for that user
Use the users Cloud password and email address for Auth
If you see
Send-MailMessage : Unable to read data from the transport connection: net_io_connectionclosed
You need to create an Authentication profile with 2fa disabled , and apply it to that user via Application Settings
Trackback from your site.