What if a landlord does not properly hold a security deposit?
If a landlord does not hold a tenant’s security deposit in the proper type of account or fails to give the tenant the required notice of where the funds are deposited, the landlord forfeits (loses) the right to keep the security deposit. The landlord, at the request of the tenant, must immediately return the security deposit to him or her.
http://www.mass.gov/courts/selfhelp/housing/sec-dep-holding.html
