I sent a letter by certified mail and received a green card receipt from the USPS stating it was received and signed by an agent. A week later, the USPS left me a (missed) notice that said the certified mail piece was being returned by the addressee re:Return to Sender. Is it legal to sign for a certified letter and then return it to the sender? The letter was sent as a first step in filing for small claims court. Thanks!

Doesn't make any difference, as far as you are concerned. The defendant received it; nothing says that he or she has to read it, or even open it. What defendant does with it after receiving it is none of your concern.

File the green receipt card with the court to prove that the defendant was served, and proceed with your small claims suit.

Who are you suing and what type of agent signed for it.

If you mailed it to a mail box store address and the store signed for it, but discovered they no longer have that client, it may be returned.

So is this mailed to a business, or a agent representing a bueinss

I agree with both answers. The nature of the agent will be the key here. If the agent can state that the addressee never received the letter, then its not proof of service.