Valediction
Let's get one thing clear. If you say "cheers" and are not British then everyone thinks you are philistine. What? Are you rejecting your own culture in favor of our old fascist oppressors?
xo -- Are friends? Lovers? If not, no.
Sincerely, -- Fine, standard, boring.
Thank you, (Thanks,) -- Make sure you are actually thanking them for something or you just look needy
Thnx, -- Are you twelve? Did you write this email in gel pens?
Farewell -- No.