Because you know what I can do

I'm getting used to all the ways people used to sign their letters: cordially yours, your humble servant, your obedient servant (as if), but the one that comes up occasionally (because I work with historical documents) is this:

Dear Mr. Doe;
Blah blah blah

Believe Me,

John McJohn

I love it. It sounds vaguely threatening. Like it really matters that you pay attention to what his letter is saying. I think I may adopt it as part of my email security signature. Yes I will. Believe me.