They will be able to increase their value by $500,000 in 1 month just by sending out a rent raise letter of $10 per unit per month.Do this. 

You can get access for free with a BP Pro plan to try it out. https://inveloapp.com/bigger-pockets/As for straight mail houses, you can check out Open Letter Marketing.

I've gone back and forth between post cards, yellow letters, professional letters, etc. and have been pulled in different directions with different mailing companies. 

You need a pitch to win in a personal letter.

You are terminating their lease, which is done without court involvement.I recommend you present the tenants with an introduction letter as soon as you close on the property.

Is there a legal letter I should send?

I'd consider starting with letters then perhaps following up with a few rounds of postcards.

These all require applicants to submit an acceptable Letter of Explanation addressing each specific issue and occurrence.Rental HistoryBecause a current landlord may say anything to get rid of a bad tenant out of desperation, we also require information for the previous landlord of all applicants.

You could respond in  a letter with those points and their options. 

We will provide a letter stating that our entity has been making payments on loan number ## in the amount of $$$ for the last X years, on our letterhead.