A very helpful information in working out the word »Malice«. But, what else, than the quoted film, could be relevant in finding out, who is behind that clearly documented abusement for so long time?

Oh, that was a crap movie :)
If the abuse is clearly documented my comment is crap like the movie, but it is beyond my understanding how someone can hack you mailbox and drop your mail. I guess you know better than me that if your password is strong enough, and your mailserver not full of holes only an insider would be able to hijack your mail.
Care to share the proof? Maybe someone will be able to help. Maybe me.

No movies, in my social education, ever helped to enflavour reality. — Actually I prefer to dream, instead of getting insulted by visualized books fazing me.

I will not share the Proofs at this time, for legal reasons. I will instead, when I can proof somebody elses criminal intent technically, finding finally the guy or whoever is doing this.

I myself can not explain why only selected mails do not reach my mailaccount, as most of them instead do normally. — Interesting the fact, that time by time some mails do not. Interesting as well the fact, that 99% of these mails are or concrete work offers, or economical relevant information, which would help on my life-way.

Theoretically this is not possible, as due to a save password.— I must assume that my password is not save enough. Practically I can and will proof a clear abuse on my mailaccount from third parties.

If this still seems a parabahavior, I will continue to notice abusement, hoping that time and someones help will bring this more than criminal act up to a legal daylight.

As I don't really care about the content of the mails, or the effect the actions of the cracker have on your life, I will instead focus on the technical aspect of this (you asked what could be relevant in finding out, who is behind this).
It sounds like you have a Trojan on your Mac, because it is not really likely that someone hacked your mailserver (as I sad before, unless it is not properly patched). If you really have a Trojan on your local box, the consequences for your privacy would be far worse than some dropped mail. The cracker would be able to collect a whole lot of personal information on your browsing habits, your contacts, your online identity, and everything else you do on your box (but I guess you know this yourself). If that's the case it could be someone that doesn't like you, or maybe someone that obsessively loves you :)
You can get rid of the Trojan youself, just have a look at the processes that are running, and look for something strange. Maybe something jumps at your eye.
But since you don't want to share the proof, everything I wrote here is just speculation. I hope you get rid of that nasty guy.