SMTP authentication using OAuth token on Azure servers is really long, due to high latency responses from the SMTP server (probably to prevent brute-force attacks).
Indeed, a AUTH LOGIN command is sent first, and have to wait for about 5 seconds get the error response back. Then a RSET command is sent and also have to wait for about 5 seconds get a response back. The AUTH XOAUTH2 command is then sent and all is fast after that.

Adding the ability to override default eSMTP authenticators will permit developers to explicitely define that only XOAUTH2 authenticator has to be used, to prevent high latency in SMTP authentication.

I will update the documentation once new methods signatures will be validated.