If you write a routine business letter, an acceptable form of address is "Dear...". This word is neutral, expressing politeness, and after it add the name, name and patronymic, the word "colleague", "comrade" or "sir". In the last three cases one must add the name of the person.
In case of a natural person, whose name you know, you should use a salutation like "Dear" and add the name of the person or use "Dear" with the addition of the name of the addressee. The degree of your intimacy with the person who receives the email will determine the time it is accessed.
If the letter is addressed to a legal person, the name can be omitted, and select one of the following addresses: "Dear sir", "Dear Mr. editor", etc. To the judges, decided to contact "Your honor."
Addressing the honored worker of arts, science or official, you shouldn't use everyday "Dear", and it is better to begin a letter with "dear" or "Respected", certainly adding a name and a patronymic of the addressee.
The appeal of "Citizen" is appropriate in the letter to the person as the subject of civil legal relations.
Turning to the collective addressee, use the expression "Ladies and gentlemen", "ladies and gentlemen" or "Dear colleagues".