When communicating with donors how we talk with them, especially in mass mailings, is really important. Some people really appreciate formality while others want you to tell a compelling story with simple language. A good general rule is the more that you know someone the more informal your writing can be.
When writing a solicitation letter if you have never met the individual it is a respectful practice to address them as Mr. or Mrs. I’ve seen many organizations that do this in the address block no matter how long they’ve known the donor. I have found that generally individual donors respond better to colloquial language than a formal request. Using conversational language makes an individual feel like they are being address personally instead of as one of hundreds of people.
Today a well-written informal letter often will get further than a formal won. If I see Mr. in a letter I receive I throw it away almost immediately.