When you send an empty email to a Roundup Issue Tracker
it adds an empty message to the issue in question

It turns out that when I'm sending the mail, Mozilla is
adding four empty lines to the end of the message.
 
In turn, Roundup is picking these empty lines up as
content.
 
The problem line is (mailgw.py, line 842, maint-0-5
branch):
 
To fix, I changed

if content:

--->

if content and not content.isspace():