Secured mail with Gmail and Postini

Message security service sends your outbound messages to recipient mail servers depending of your option :

Send only SMTP: No TLS encryption, and all messages are delivered via SMTP.

Send by SMTP or TLS (Recommended): This is the recommended setting. Messages sent via TLS are delivered via TLS to the recipient. Recipient servers that do not support TLS will receive their mail delivered via SMTP. All other messages are delivered via SMTP.

Send by TLS if possible: This delivers all messages by TLS when possible. Recipient servers that do not support TLS will receive their mail via SMTP.

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.

Source : http://www.postini.com/webdocs/admin_msd/wwhelp/wwhimpl/js/html/wwhelp.htm?href=logsearch.20.6.html#2976107