Massachusetts General Laws state that for motor vehicle infractions or violations a law enforcement officer may issue a written warning or cite the violator. This section of the law does not say how a warning ticket will affect a MA or out of state driver. It does appear though the a written warning is not at least in your MA record since it notes that 3 written warnings within a 12 month period may result in penalties.
Maine is part of the Drivers License Compact (DLC), which requires member states to report back convictions of traffic tickets the state which the motorist is licensed in, while Massachusetts is not. However Massachusetts still has agreements with other states to exchange ticket information.
The motor vehicle laws of Maine discuss the DLC. In this section of law it states that the licensing authority of a party state shall report each conviction of a person from another party state occurring within its jurisdiction to the licensing authority of the home state of the licensee. The report must clearly identify the person convicted; describe the violation specifying the section of the statute, code or ordinance violated; identify the court in which action was taken; indicate whether a plea of guilty or not guilty was entered, or whether the conviction was a result of the forfeiture of bail, bond or other security; and must include any special findings made in connection with the conviction.
It also notes that the licensing authority in the home state shall give such effect to the conduct as is provided by the laws of the home state. However in your case it does not appear as if you would need to worry about this MA infraction going against your ME license since it was a warning and not a ticket to which you were convicted of.
Even if this had occurred in state, in our experience, in most cases a written warning does not normally affect your insurance because it does get sent to your record. There are exceptions where it can appear on you record, in those cases the infraction would affect your rates. For a Maine auto insurance quote click here.