Microsoft 365 Exchange Error Code 5.7.321 – starttls-not-supported: Destination mail server must support TLS to receive mail.

In this post, let’s have a look at the error message “5.7.321 – starttls-not-supported: Destination mail server must support TLS to receive mail.” that you get when working with Microsoft 365 Exchange Online.

These are the errors that you usually receive when there is a problem in delivering the email message that was sent by you. These errors are generated and sent by Office 365. This is usually alled as DSN which stands for delivery status notification or in simple words bounce messages.

NDR or Non-delivery report is one of the common type of message that you receive which indicates to the user that the email was not delivered.

Error Message

5.7.321 – starttls-not-supported: Destination mail server must support TLS to receive mail.

Error Details

DNSSEC checks have passed, yet upon connection the destination mail server doesn’t respond to the STARTTLS command.
The destination server responds to the STARTTLS command, but the TLS handshake fails.

This message usually indicates an issue on the destination mail server. Check the validity of the recipient address and determine if the destination server is configured correctly to receive messages.