english

Formal letters should close with 'Sincerely yours', 'Yours sincerely', or (in BrE) 'Yours faithfully' if the name of the recipient is unknown.

There are other closings, but those are universally accepted for formal letters.

In American English:
Sincerely,

In British English:
Kind regards,

Of course there are other ways as well. Just Google some templates and take a look.

ElanguestIn American English: Sincerely,

This is considered a tad rude by some: not bothering to finish the greeting puts some readers off.

Hmm, it's perfectly acceptable in America.

ElanguestHmm, it's perfectly acceptable in America.

If you mean 'Sincerely', not to some; I'm American. It is OK from a higher status position.

Really? I have never experienced any problem with it. In fact, it's how I learned to end a formal letter when I was in school.

ElanguestI have never experienced any problem with it

I haven't either. It is just that some think abbreviating the full complimentary closing does not show full respect.

I never understood it as an abbreviation of sincerely yours. I always understood it to mean that the letter had been written in an attitude of sincerity.

ElanguestI never understood it as an abbreviation of sincerely yours. I always understood it to mean that the letter had been written in an attitude of sincerity

And your interpretation is a good one; I'm just stating what some readers think.