Support for STARTTLS in SMTP Proxy was added after build 5776. Please refer to http://www.xeams.com/changelog.htm for actual build number and dates.

Besides STARTTLS, several other features have been added since build 5776, which are increasing becoming standard and eventually receiving servers will penalize the sender if they are not used. For example, gmail will warn/notify if STARTTLS, SPF, DKIM, and DMARC are not used.