Open vs. Mixed Punctuation

When using open punctuation, no punctuation is used after the greeting or the complimentary closing.

Ex.

Dear Ms. Smith (greeting)
{BODY}
Sincerely (closing)
When using mixed punctuation, there is a colon after the greeting and a comma after the closing.

Ex.

Dear Ms. Smith: (greeting)
{BODY}
Sincerely, (closing)