It is common to use a colon at the end of the salutation of a letter. A comma is also acceptable, but a colon is more commonplace and looks more professional.
The salutation is followed by a double space before the first body paragraph. The salutation should name a specific person if available. Salutations such as "To whom it may concern" or "Dear sir" are outdated. The salutation should directly address the specific person to whom the letter is addressed whenever the name is available to the letter writer. This gives the reader a glimpse at the writer's level of commitment and shows a level of professionalism.