This letter thanks a colleague for a referral.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I felt I should send this letter immediately to thank you for your kind referral of <client name>. It's gratifying to see that we can work together to bring each other business.
<description of referred business>. I made sure to let <referred contact name> know that your referral was the reason for our meeting.
Thank you again for the referral, and I hope I can do you the same service very soon.
encl: <List of enclosed items goes here>