Google Message Security for Google Apps Administration Guide - Manual (Page 302)

302 Message Security for Google Apps Administration Guide
2. Choose a setting for TLS delivery from the message security service to
Google Apps Mail. Select “Send by SMTP or TLS” which provides the
greatest flexibility.
Following are descriptions of each delivery option:
• Send only SMTP
No TLS connections from the message security service to Google Apps Mail.
If a message is sent via TLS, it is received by the message security service in
encrypted form, but delivered to Google Apps Mail unencrypted via SMTP.
• Send by SMTP or TLS (Recommended)