If you wish for more than one person at that address to read your letter then you should address a letter and envelope, separately to each person. As a courtesy, you can list all the recipients' names at the bottom of your letter, after your name and beginning with 'c.c.' then followed by the people who received a copy.
Emails, being a less formal way of writing, usually begin with 'Hi', or just the persons first name. If you wish to send to several people,they are copied in the original email address bar. The subject line is sometimes more important than the salutation because it can determine where the email is filed, who it is forwarded to, and whether it has immediate priority.
Fax Letters have a different method of address, and either use a separate cover sheet or a header with blank fields for filing in: the recipient's name, department and fax number; your name along with your contact information; the number of pages being faxed; the names of the people who are being copied on this letter.
Strive to have your salutation addressed to a specific person instead of the generic Sir/Madam. Search company or government websites to find the correct title, department and spelling of their name, but don't rely on this information entirely it may not be current. Phone their customer service desk to verify if that person still holds that position or has someone else taken his or her place. The recipient will appreciate your attention to detail and will hopefully read your letter with a more positive attitude that could bring about favorable results