Google Message Security for Google Apps Administration Guide - Manual (Page 320)

320 Message Security for Google Apps Administration Guide
This message is an automated alert from your email security
service.
Your email security service was unable to send messages to the
following domain, because your outbound mail server did not
establish a TLS connection to the service:
<recipient domain>
Your Outbound TLS by Domain encryption policy requires this domain
to receive messages using TLS. The email security service defers
messages if either your mail server fails to use TLS, or if the
service cannot establish a TLS connection to the domain's mail
server.
Recommended action: Verify that your outbound mail server supports
TLS connections. Then check your TLS settings in the email security
service's Administration Console.
This problem most often happens because you enabled Policy Enforced TLS in
the Administration Console but did not enable TLS in Google Mail. Contact
Google Apps support to set up TLS for your Google Apps Mail account. For
information about how to contact Google Apps Support, see the Google Apps
Admin Help Center.