Exchange Online speichert ab 01.06.2018 per SMTP versandte Mails in Sent Items
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.