– Failed to get initial SMTP Response

If something on the client-side is blocking communications, such as a hardware or software firewall, or anti-virus, then it may be that a TLS connection is established, but then trying to read the 1st message from the server gets blocked.  When this happens, the contents of the LastErrorText look like this.   (The LastErrorText for a successful connection is shown below.) […]

SMTP / Office 365 / OAuth2 : SmtpCmdResp: 535 5.7.3 Authentication unsuccessful

Question: A customer wants to send emails with Office 365 and OAuth2 Authentication. The setup of the application in Azure is OK. It works for one user. For another user we have the following error: SmtpCmdResp: 535 5.7.3 Authentication unsuccessful [****.****] For authentication I use the example: What is surprising is that for the second user on same PC, […]

Office365 SMTP Error: 554 5.2.0 STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied;

Question: I’m trying to send email using SMTP via, and I get this error: 554 5.2.0 STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied; Failed to process message due to a permanent exception with message Cannot submit message. 0.35250:0A001485, 1.36674:0A000000, 1.61250:00000000, 1.45378:02000000, 1.44866:59020000, 1.36674:0E000000, 1.61250:00000000, 1.45378:5E020000, 1.44866:F8000000, 16.55847:BC030000, 17.43559:0000000024020000000000000000000000000000, 20.52176:140FFA8B1500001000000000, 20.50032:140FFA8B8517001000000000, 0.35180:F1030000, 255.23226:09000000, 255.27962:0A000000, 255.27962:0E000000, 255.31418:F1030000, 0.35250:0300C981, 1.36674:0A000000, 1.61250:00000000, 1.45378:02000000, 1.44866:3A000000, 1.36674:32000000, 1.61250:00000000, 1.45378:3F000000, 1.44866:01000000, 16.55847:A6000000, […]

Authentication Problem with Sending Email in Office365 using SMTP

If SMTP authentication (using login/password) fails for your account, then it may be that your account is configured to require MFA (multi-factor authentication).   You may need to change settings to allow for legacy authentication (single-factor auth).   See Also, an app password may be required.  See