What to do when certified mail is returned - deposit return

[Nick Ferguson]

So I had a group of tenants who were college students.  Five of them in total.  They all moved out and went separate ways.  They are NOT getting any deposit returned and, in fact, owe a little over $1,000 in damages that exceeded the deposit amount.  I'm not really counting on ever seeing any of that but I did have my attorney draft up a letter saying "pay this or else" and sent them the explanation of the security deposit via certified mail as required.  

One of the five tenants has not responded to my numerous inquiries regarding a forwarding address so his was actually never sent but I have documented numerous attempts to contact him via phone and text.

The real question is this.  Two of the letters were just returned to me as undeliverable because the tenants were not there to accept the letter and did not respond to the post office notice and the letter was returned after 3 failed delivery attempts.  

What am I legally obligated to do?  Have I already done my part since I tried to send?  Do I need to reach back out, etc.? 

Beyond what I'm legally obligated to do, what CAN I do to attempt to persuade them to pay the amount owed.  I'm not going to go through the hassle of small claims court for such a small amount but I assume I can send them an email or text message threatening small claims court even though I have no intention of following through. 

Any advice is appreciated.