So it turned out that having "Receive Private Messages" set to no also means you cannot "send" PM's. Interesting.
However, while I was testing it seems that the PM sent from the follow user mod does not show the signature. The normal PM's I send from the bot does.
Ok cool I have some info to go on now, I'll do some testing and report back when I have fixed the issue, probably some time tomorrow. I should be able to fix the signature thing too.
Can you pm me with a admin account so I can see this in action for myself, maybe I might see something that could cause that. If you have a test account also that would be helpful so I can follow and post and see the pm that is sent.
I still can not get it to do that even on your site, here is the steps I took.
1) I added a new test account "ozzy test"
2) I as the admin account you sent me followed the above user.
3) I as the new account posted in the forum.
4) I as the admin account got a pm about the new post.
Oh is that all that is left is the signature? I though you were still having issues with it sending the pm with the Receive Private Messages set to No. Which I have set to now.
As far as the signature that will be added in the next release with a option to show it or not in the settings. I should have that out sometime this week if all goes to plan.