Rent Increase Issue - Massachusetts

[Dan Smith]

I'm in an interesting dilemma here:

Quick backstory: I bought a three family in Massachusetts a little over a month ago.  I inherited tenants on the first floor.   Before actually closing on the house, I drove out to introduce myself to the new tenants to show them that I'm a real human being and go over changes that would take place for them.  They currently pay 850/mo, they have been paying that for 10 years.  I told them that the rent will be increasing to 1200/mo and i wanted to let them know as far ahead as possible, in case they felt that they couldn't afford it and needed to find a new home.

On the day of closing I presented them with a letter stating the rent increase to 1200, this was dated December 16 and told them that as of February 1, their rent was to go up.  

Fast forward to yesterday (January 8), they tell me that they don't have to pay the increased rent because I never sent the letter via certified mail, (their lawyer friend apparently told them this).  I am obviously frustrated as I feel that they are trying to be slick here.  I let him know that it didn't have to be sent certified because I gave it to him in person, but he insisted he was correct.

I am curious what my options are to educate them that they do indeed have to pay the increase and that me presenting the letter in person was appropriate?

Also, If I am wrong, I would love to use this as a learning experience and see what other info some may have.

Let me know if I'm missing any details here to make the situation easier to understand!