When configured to use opportunistic TLS, Messaging Gateway (SMG) will attempt to fall back to plaintext delivery when the TLS communication attempt fails. In some cases, this fallback to plaintext delivery is not occurring which can result in messages getting "stuck" in the delivery queue.
451 4.4.2 [internal] send helo/ehlo failed
This issue has been addressed with the Messaging Gateway 10.6.2 release.
Please see the Messaging Gateway Administration Guide for detailed instructions on how to update the SMG software.