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.

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I like this post because the secret is in the details when dealing with people that you want to fund your organization. I to tend to read with much more curiosity and acceptance when addressed by my first name as a friend or neighbor might call me.
Thanks for reminding us of the importance of warming a letter up!