The confusing server error *User is authenticated but not connected* might occur if:
The confusing Microsoft specific server error *User is authenticated but not connected* might occur if:
* External access was administratively disabled, please see [this article](https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/client-access-rules/client-access-rules) about how an administrator can enable it again
* SMTP was administratively disabled, please see [this article](https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission) about how an administrator can enable it again