From code inspection:
The receipt will only be send automatically if (1) the return path is empty, and (2) it is equal to the address the receipt should be sent to.
Obviously, both can not hold a the same time. This seems like a conversion error: http://bonsai.mozilla.org/cvsview2.cgi?diff_mode=context&whitespace_mode=show&root=/cvsroot&subdir=mozilla/mailnews/extensions/mdn/src&command=DIFF_FRAMESET&file=nsMsgMdnGenerator.cpp&rev2=1.21&rev1=1.20 around line 371. The check should be reversed.
Now, I can't test this bug, because my server messes with the return path. So it will never match for me anyway. And I don't know if this should be fixed at all. People might have unknowingly set the pref to autosend, but still got asked to send, because of this bug.
Created attachment 419143 [details] [diff] [review]
This patch should fix it. But see my remarks in comment 0
this bug was almost certainly introduced in 3.0 so I think fixing it is the right thing to do. Thx for the patch.
Comment on attachment 419143 [details] [diff] [review]
this does fix the problem for me.
We've had several reports of this...
I pushed this to trunk so we can get a little baking time before we want it on branch:
Patch checked into 3.0.1pre in time for 3.0.1: