Mixed punctuation uses a colon after the salutation and a comma after the complementary close.
Example:
Dear [Name],
[Body of the letter]
Sincerely,
[Your name]
Note:
* Open punctuation does not use any punctuation after the salutation or complementary close.
* Closed punctuation uses a colon after the salutation and a period after the complementary close.
However, the use of mixed punctuation is becoming less common, and many style guides recommend using open punctuation instead.