Firing my PM but certified mail being returned undelivered

[Liz Norman]

I have sent certified mail with 60 days notice to terminate my contract with my property manager. I sent notice to my tenants of the change at the same time. The Property Manager has recently moved offices, and when I asked for the new address he didn't respond. I reached out to his partner, but it looks like the address he gave me is incorrect and tracking shows after attempting to forward the letter it is coming back to me. Tenant's notice letter was delivered. Now I am not sure what to do. This seems to screw my needed 'necessary notice' window, and there's a good chance my tenants will ask the PM about the change, who at this point doesn't know they're getting fired. If the notice comes back to me what do i do? Stick the whole thing still in the envelope back into a newly addressed certified letter to maintain original postmark? Does this proof of notice date cover me?