A proof of residency letter from the landlord provides proof that the resident resides at their specified address.
If the letter is being sent as a hard copy, then it should be in the proper business format, otherwise, if the letter is being emailed, it should have an appropriate subject line and proper grammar.
My name is [Your First and Last Name]. I am the landlord of [Apartment Complex], located at [Apartment Complex Street Address, City, State, Zip Code]. I'm sending you this formal letter as proof of one of my tenant's residency. [Tenant's First and Last Name] has lived at [Apartment Complex], [Apartment Number] since [Day, Month and Year your tenant first started living there.]. [His/her] rent is [Dollar Amount of Rent] per month.
If you have any questions, feel free to contact me by phone at [Phone Number], or by regular postal mail.