This letter informs a company about the actions of a competitor.
1234, Main Street
Boston, MA 02123
31 March, 2005
<Recipient Address Goes Here>
I have been a customer of <company name> for <time you've been a customer>. I have nothing but good things to say about your company, so I thought I should let you know about <competing company name>, a competitor of yours in <description of market>.
I like your company and the work you have done for me and others, and I thought I should inform you of the actions of your competition. If you need more information, feel free to give me a call.
encl: <List of enclosed items goes here>