Use a more formal essay format. For example:
Begin your letter with either Dear Ma'am or Sir; OR To Whom It May Concern;. If you know who exactly is going to be reading your letter, you may title it to them, however it is considered more formal to address it as stated above. An extremely formal letter (such as a thank you to a grant foundation) would read:
To Whom It May Concern:
On behalf of....I would like to thank you for....The grant was much appreciated and benefited...We are immensely grateful for your kindness....