"504 5.7.4 Unrecognized authentication type" when using Office 365 SMTP relay

This error occurs when there is a mismatch between the TLS/SSL settings in MultiSendcon and what Office 365 requires.

Fix

In the MultiSendcon SMTP account settings for smtp.office365.com, change the SSL option to "Explicit SSL/TLS".

Note: Explicit SSL/TLS support was added in MultiSendcon version 2.10. If you have an older version, download the current version first.

Office 365 requires STARTTLS (explicit TLS on port 587). Plain SSL (implicit, port 465) is not supported for outbound relay through Office 365.