Google Message Security for Google Apps Administration Guide - Manual (Page 305)

Transport Layer Security 305
2. Choose the Outbound TLS settings. To do this, you must select how the
message security service accepts outbound messages from Google Apps
Mail, and also how the message security service sends your outbound
messages to recipient mail servers. These settings are described below.
Choose “Accept SMTP and TLS” which provides the greatest flexibility.
The first Outbound TLS setting enables you to choose how the message
security service accepts outbound messages from Google Apps Mail:
• Accept only SMTP: With this selection, the message security service
delivers messages only via SMTP from Google Apps Mail. Requests for
TLS connections are refused.
• Accept SMTP and TLS (Recommended): With this selection, the
message security service accepts messages via either SMTP or TLS
from Google Apps Mail.
• Accept only TLS: With this selection, the message security service
accepts message only via TLS from Google Apps Mail. All non-TLS
encrypted messages are deferred.