Google Message Security for Google Apps Administration Guide - Manual (Page 298)

298 Message Security for Google Apps Administration Guide
This diagram shows the flow of TLS messages between servers:
• Stage 1: The sending server sends a message via TLS to the message
security service, which always accepts TLS messages and process them
according to the TLS protocol. The message is encrypted from the sending
server to the message security service.
• Stage 2: You can choose whether the connection from the message security
service to Google Apps Mail uses TLS.
You may choose not to use TLS if you wish, but this will reduce security. You
receive the message from the message security service unencrypted via
SMTP.