Seems to be somehow related to the reply stripping feature in MDL-48002 - perhaps caused by recent changes since we only discovered this after upgrading to 2.9.1.

Steps to reproduce:

1. Enable and configure the reply to email feature.
2. Post a forum post
3. Reply to that forum post using gmail

Expected results:

1. The reply is posted on forum

Actual results:

1. A blank message is posted

I have added a fixture to this issue which demonstrates the issue.