In Dynamics 365 Business Central Support we regularly receive questions / concerns about how you should configure the SMTP Server component in D365 BC.
A good start is of course to run the Assisted Setup wizard. Or you search for SMTP and click on the relevant link in D365 BC. Here you can apply the default settings which by default configure Dynamics 365 BC with the Office 365 SMTP server. This of course does make sense as for the authentication of D365 BC we do require an AAD account or Office 365 account.
The account specified here, must be added to each Office 365 user in the Send As permission page within Mailbox Properties. You then allow each D365 BC user to send as the SMTP user account. This does require an Exchange Online license to be added to the users that do need a mailbox associated with their account.
Hope it does help you further.
