When we use a colon after the salutation and a comma after the closing, we refer to it as having open punctuation.

Name: When we use a colon after the salutation and a comma after the closing, we refer to it as having open punctuation.
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Using a colon after the salutation and a comma after the closing is referred to as mixed punctuation, not open punctuation. Open punctuation typically omits the use of punctuation after the salutation and closing in a letter.

When We Use a Colon After the Salutation and a Comma