Why there are 2 versions of the Phantom Antichrist Box Editions existing?
One ist the Mailorder Edition which is a metallic box and the Ultimate Edition which I have. It’s both the same only the embossed thin metallic cover is the difference and the limitation of 500 pieces instead of 1000. Why I bought the Ultimate Edition and not the Mailorder Edition?

There 2 versions indeed.
The Mailorder Edition has the tinny metallic embossed attached to box, that’s limited to 500 numbered copies. This version was exclusive available at pre-order through Nuclear Blast mailorder (it got sold-out 3-4 weeks before street date).
The Ultimate Edition is limited to 1000 numbered copies and does not have the tinny metallic embossed cover.
The contents of both boxes are the same (as far as i know).

Correct, the contents are identical.

Yeah.  Totally the same.  I have both.