I am looking at proxying to google as well, and thus need SSL on the backside (and would like it on general principles for other cases as well), however it does not appear that nginx supports this. I would expect this to be the default if an incoming connection is using ssl, or, at the very least, specified in the protocol parameter in the server {} block (e.g. pop3s, imaps, smtps or smtp-starttls). Neither appears to be the case, nor do I see any options that suggest the ability to enable it in some other way...