How do I give back a rental security deposit refund

Rental security deposit refund disputes are the number one reason for post-tenancy litigation

Here’s a great question that we got from a landlord recently. The reason it became an article is because of the potential problems a rental security deposit refund can cause. To see what I mean, check out this quick news article about a landlord who was court ordered to pay double damages because of a security deposit dispute.
My tenant recently moved out of their apartment, but did not give me their new address. I have their security deposit that needs to be returned to them, and am not sure what to do with it, any suggestions?
There is a service that the US Postal Service offers, that tells them what to do if the mail happens to be undeliverable as it is addressed. The service is called ancillary service endorsement, and it allows them to forward the mail to a new address that they may have on file, or return the undelivered mail to you, the sender so that it does not get lost.
So for your position, I would address the envelope with the address that they had when living at your apartment. Then, assuming they have already filed a change of address with the post office, they will receive the check at their new address.

A good rule of thumb: Make every reasonable effort

Key word being reasonable. You don’t have to go calling nearest relatives and hire a PI to track the tenant down, but certainly make a documented effort to return the money and do the right thing.
Landlord Tip: Send a letter requesting the tenant’s new address before you try sending the check to return their rental security deposit. For obvious reasons, including the fact you don’t want to knowingly send money to a wrong or unverified address