Google Message Security for Google Apps Administration Guide - Manual (Page 307)

Transport Layer Security 307
• Send only TLS: Send all messages by TLS. Mail sent to recipient servers
that do not support TLS will be deferred.
• Send and Deliver TLS: Messages sent via TLS are delivered via TLS to
the recipient. If the recipient does not support TLS the message will be
deferred. All other messages are delivered via SMTP.
3. If you wish to set up specific domains for TLS, enter domain names in Domain
Specific Settings. See “Setting Up Policy Enforced TLS” on page 311 for more
information.
4. Click Save.
About Policy Enforced TLS
Use Policy Enforced TLS when you need to make sure that all mail connections
with a specific domain are encrypted. Usually, Policy Enforced TLS is intended for
contacts or partners with whom you regularly exchange sensitive mail that must
be encrypted.
Policy Enforced TLS defers all unencrypted messages to or from a particular
domain.