Give the candidate's name and the position he/she is seeking. Be specific about the type of information you're seeking.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
<Name of candidate> has applied for the position of <job title> with our company, and has listed you as a former employer. Could you provide us with verification of <his/her> employment, including dates of employment, job responsibilities, performance rating, salary, and reason for departure?
Any other impressions you could provide as to <his/her> suitability for this position would also be appreciated. Of course, all information you provide will be kept confidential. Thank you.