Should you use a comma/period after “Thanks”/“Regards” in email signatures?
When writing a letter, do we put a comma or a period after the valediction?
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The comma is the only acceptable punctuation to place after the valediction:
This is because the period is only necessary at the end of a sentence or abbreviation. The comma, on the other hand, usually separates related phrases within a sentence. In this case, the context is not a sentence (Sincerely, John Doe), but convention has established the usage of a comma in a letter's closing in much the same capacity as in a sentence.