Mailing lists are sacred property to most companies, but people are often willing to give access to their lists if 1) they can make money selling the names, 2) they can access your list or 3) it is a joint marketing effort. Be clear about what you will do with the list and what the benefit is to them.
1234, Main Street
Boston, MA 02123
08 April, 2005
<Recipient Address Goes Here>
We would like permission to access your mailing list. We are <intentions>. We would <how they would be compensated>.
I will be calling you <when you will call> to see how you feel about this. If you have any questions, you can reach me at <phone number>. I look forward to hearing from you.
encl: <List of enclosed items goes here>