1.6 FORGED_MUA_MOZILLA Forged mail pretending to be from Mozilla

[Jerry Malcolm]

I am sending test emails from one of my hosting environments to another 
of my hosting environments.  I get this line in the SA report:

1.6 FORGED_MUA_MOZILLA     Forged mail pretending to be from Mozilla
I am sending from Thunderbird.  So it's coming from Mozilla.  But it is not 
forged mail pretending to be from Mozilla.
What is triggering this?

Thx