[NAME, COMPANY AND ADDRESS, ex.
John Smith
XYZ Inc.
1234 First Street
Suite 567
Anycity, Anystate  85245] 

Dear [NAME, ex. John Smith],

It is my pleasure to inform you of a new addition to our sales staff.  ["TOUT" NEW EMPLOYEE, ex. William Holmes brings with him five years of experience in the computer industry; he has extensive knowledge in small-business intranet systems and I am certain you will be pleased with him.]  [STATE HOW CUSTOMER WILL INTERACT WITH NEW EMPLOYEE, ex.  William will be pleased to service your account and serve you to the best of his abilities.] 

William can be reached directly at XXX-XXXX. 

[He/She] is looking forward to hearing from you.

Sincerely, 

[YOUR NAME, ex. Jill Jones]