In a word, probably "YES".
As with all things in real estate, it is governed by the actual listing agreement you signed with your real estate agent or broker. If your real estate agent is a RealtorÂ®, then they are probably using the standardized listing documents provided by their Board of RealtorsÂ®.
The Greater Hartford Association of RealtorsÂ®' listing contract requires (as do most standardized listing contracts) that listings be input into the Multiple Listing Service. The Connecticut Statewide Multiple Listing Service rules require that to be completed within 48 hours of all signatures on the listing document.
In order to be exempted from the requirement to submit the listing to the Multiple Listing Service, the homeowner must sign a separate form which states he understands that the MLS is considered an essential service and that failure to use that service may affect the results you are trying to get in selling your home.
On the other hand, I would have to ask HOW you know that the property is not listing on the MLS, since that particular database is proprietary and only certain people have access to it. While Realtor.com and ctreal.com are the public sites which receive the MLS information, agents have the option to disclose or NOT disclose the address of the property, so if you are searching for your home on these sites by the address, it may not show up in your search, and it may actually be there.
If you have any further questions about your situation, you should first talk with the BROKER of the agency that has your listing. After all, each of the salespeople for that company actually work under the Broker's license, and it is the Broker who is ultimately responsible for the contractual agreements you have made with their Authorized Representative (agent).
Lisa Orme, Broker
The Master's Key Realty, Windsor, CT