Technically business letter closings is known as "Complimentary close". The following are some of the most common phrases for a formal, professional or friendly Complimentary close.
Formal: Sincerely, Cordially, Respectfully yours, Very truly yours, Respectfully
Professional: Regards, Kind regards, With warm regards, Yours truly
Friendly: Best wishes, Warmest wishes, All the best, Thanks for your help.
Here are some examples that can be used:
To take action or to highlight a critical point:
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597 Business Letters - Software