HowTo Test a Mail server BEFORE going live

Use the Wormly website to test email deliver either before going live with at new mail service or if there are problems.

https://www.wormly.com/test-smtp-server

If you are setting up a non-profit using Microsoft Office 365 non-profit free service, you need to “Purchase” the licenses for $0 and assign them to users before you can deliver any email as listed above.

Side Note:
Microsoft does not allow email forwarding until you turn it on
https://office365itpros.com/2020/11/12/microsoft-clamps-down-mail-forwarding-exchange-online/