Doctors hand out those letters to anyone that ask.

It's like reading a newspaper article on diabetes, and then telling your doctor he's wrong when he diagnoses you with it.

This would have to be sent from a company specific email or fax that I can reference online so the tenant isn't doctoring the form.

I have an emotional support animal, and my doctor has written a letter saying why such an animal is beneficial to me. 

One has a great credit score, one has a rich father (Doctor) who pays his rent no matter what, and two have zero credit history, and both have erratic or very recent job hires.I've already rejected one of the four due to no credit history and too short of job history, and the fact that the guy doesn't respond when I asked for follow up info. 

I would then read the document to see if the doctor included text that specified that only smoking the drug would achieve the desired result. 

For example, a doctor wouldn't refuse to treat a serial killer, so transitively it would be bad business for a landlord to refuse to rent to a serial killer.Do you agree with that statement? 

Your tenant with heart issues might be able to find a doctor to classify him as "disabled" or "handicapped".

When he requested receipts, the landlord faked them and it was obvious they were doctored. 

If it's not obvious, then you can ask for proof that she is disabled - not the details, but a document from her doctor or psychiatrist that just says she is disabled will do.