This letter will work for individuals and companies alike, for business or personal reasons. Send when you want to congratulate someone on a particular anniversary.
{Date}
To: {Name}
{Address}
{City, State, Zip}
Dear {Recipient},
We here at {name of company} wish to extend congratulations to you on your {number} anniversary with {the company, a person, etc.}.
We know you have worked hard to reach this milestone, and we look forward to many more years of {what this person does}.
Sincerely,
{Sender}