I have to write an email to someone I greatly dislike, and do it politely. I’d like a closing salutation that says “Now fuck off” but in more professional language. Victorian constructions like “I am your most humble and obedient servant” might do.
“So long, you festering bog worm” lacks elegance.
Well, “Bless your little heart”, always works. IMO
“I can’t say what a pleasure it is to work with you. I really can’t.”
“Very truly yours”
Maybe “enjoy your evening” – which is something I’d use to close an exit interview with a bad boss as I walked away for the last time, free at last
Regards,
ArtBeforeScience
Honestly, if this is a professional setting, I would refrain from the snark, or you may end-up in hot water. Remember, it’s not what you intended, but how it was received.
“Please don’t feel obliged to reply”…?
Remember: never type out words in an email that you wouldn’t want read aloud in a courtroom
“I hope you have the day that you deserve.”
But in all seriousness: be the better person. This can only end up biting you in the ass.
Honorable mention to this suggestion. But:
and
and
… are sadly most likely to be the best advice in the long run.
It sucks not to vent one’s spleen to deserving targets. But thanks to Ann Landers/Eppie Lederer, my common-sense hero, I’ve mostly held back in life. You can imagine the snark, write it out in detail, but don’t send it.
Never, ever have I ultimately regretted not expressing the snark. When I have given in to the desire to give someone the lecture/snark criticism that they so richly deserved, I’ve invariably been sorry later.
Agree, it rarely works out to your benefit to bite the dog that bit you. They’ll turn on you, Everytime.
I don’t disagree, but the reason to bite back snark is not only due to personal “benefit” or the fact that “they’ll turn on you” if you express yourself.
In my case, and perhaps others who have posted, it is more about, as @purplehorseshoe says, “being the better person.”
I think this is attributed to George Bernard Shaw:
" Thank you for your correspondence. I am holding your letter in front of me, as I sit in the smallest room in the house. It will soon be behind me."
But, yeah, as others have said above: be careful. Don’t write what you don’t want published in huge headlines…
Email is forever. And it will come back to bite you .
Think of all the celebs, etc who get fired for innocent snarky comments they made 25 years ago, which are now deemed racist or whatever.
Years ago some work friends and I used to joke that “regards” was really calling someone “retards” because you could actually call them that and blame it on the auto-correct.
warmest retards,
msmith537
Start each word in the email with letters that spell out an insult e.g. Forever understanding choices kills obvious final farewells. 
There is a tale that the famously terse PM Clem Attlee once replied to an extended letter from one of his party rebels about just where and how his government was going wrong, with a card saying “Your comments have been noted”.
I get questions at work from the public. Mostly about addresses. It’s usually about a three sentence response.
I used to use
Regards,
enipla
I was rather shocked to find out that that can be insulting. Don’t understand why. Saying Thank You seems stupid since I’m doing the work, and helping them. I don’t know what to say anymore.
I had the same problem when I worked a corporate job. Some people auto-signed every email with, “Thank you,” which always rubbed me the wrong way.
Gratitude is important, and those emails cheapen it.
Anyway, my solution unless I was expressing gratitude myself was to sign off with, “Cheers!”
…had a couple folks ask if I was British LOL.
Cf. H.L. Mencken’s standard response to any opinionated letter:
Dear ***: You may be right.