503 5.7.0 encryption too weak 0 less than 128

Products

• Messaging Gateway

Issue/Introduction

You have set up a message to be sent requiring TLS but the message shows in the Message Audit Log as not delivered with the error "503 5.7.0 encryption too weak 0 less than 128"

Cause

The mail server you are trying to send to does not advertise the STARTTLS command

Resolution

This can be caused by multiple situations:

• The recipient mail server's MTA does not support or has not been enable to advertise TLS.
• A feature such as 'inspect esmpt' has been enabled on the local or remote firewall masking out the STARTTLS command.

To obtain more details about the possible resolution and settings, please refer to Knowledge Article:

Specifying SSL restrictions for TLS communications