The STARTTLS implementation in WatchGuard XCS 9.0 and 9.1 does not properly restrict I/O buffering, which allows man-in-the-middle attackers to insert commands into encrypted SMTP sessions by sending a cleartext command that is processed after TLS is in place, related to a "plaintext command injection" attack, a similar issue to CVE-2011-0411.
https://exchange.xforce.ibmcloud.com/vulnerabilities/67729
http://www.kb.cert.org/vuls/id/MAPG-8D9M75