Siehe https://blogs.technet.microsoft.com/exchange/2018/04/20/changes-coming-to-the-smtp-authenticated-submission-client-protocol/ für Funktionalität und Limitierungen.
Die Essenz ist hier:
- Sent email will now be stored in the Sent Items folder of the mailbox.
- Only three concurrent connections to our service per mailbox will be allowed. Additional connections will be rejected with the error: 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded.