Of course he can send you a letter saying that. He can also ask you to leave. However, if he were to try to actually evict you, he would probably have trouble evicting you on this set of facts as stated. For example, if the lease just encourages you to respect the right to quiet enjoyment of the other tenants (which, by the way, is a term of art that doesn't specifically mean that you must be quiet), but doesn't state that failure to do so is a breach of your lease, you could argue that you hadn't breached any explicit lease term. In addition you have the right to enter your home after you get off of work; the fact that another tenant may be able to detect your presence on the stairs doesn't mean you're being unreasonably loud. I think you'd have a strong defense to eviction, unless you have been unreasonably loud.
What I would NOT do is send a letter apologizing for being loud, promising it won't happen again, etc. or make any other oral or written statements along similar lines. I might send a letter back with delivery confirmation stating that you haven't been unreasonably loud, that you haven't breached your lease, that you can't control whether other tenants make unfounded complaints against you, and that you will consider threats of eviction without any attempt at a reasonable investigation to be an unfair and deceptive act or practice within the meaning of the Massachusetts Consumer Protection Act. You could even state that the constant unfounded reports, coupled with vague threats of unwarranted eviction, are interfering with your right to quiet enjoyment.
Hopefully, after you and the landlord get done blowing smoke at each other, he will back off a bit. This is unfortunately a problem that occurs with some frequency, and the landlord is attempting to set things right by essentially taking a stern approach. If he's doing anything really wrong, it's in being too quick to simply believe the other tenants and too quick to threaten you.
Another thing you can do is videotape yourself, with sound, going up the stairs after work and making a normal amount of noise. Set the volume at a certain level, and once you have entered your apartment, without a break in the recording, speak briefly with your boyfriend to show that the sound isn't damped. Also make sure, if possible, that the tape shows the proper date right on the recording. If the downstairs tenants complain then, you can show the landlord directly that you haven't acted unreasonably, and you could also use the tape as evidence in an eviction case.
If you evervreceive a notice to quit, consult a landlord-tenant attorney to help determine your rights. You may have counterclaims which are worth money, and your attorney's fees may be payable by the landlord. Good luck getting this situation sorted.
Your landlord probably called you so that he can now report to the other tenants that he has been proactive regarding the problem. That said, he still could try to evict you, but would have to show in court that your alleged breach of the lease was a material or significant one. All tenants have the right to quiet enjoyment irrespective of whether that term is included in the boilerplate language of their rental agreement. Just as the other tenants have a right to quiet enjoyment of their unit, you too have the right to quiet enjoyment in yours and should be free to enjoy it without unwarranted interference by your landlord. If your landlord does try to move forward with an eviction, he'll have to properly inform you of this with a written notice to quit that complies with the lease and Massachusetts law. The notice to quit is just the first step. If you don't vacate by the time stated in the notice, your landlord will be forced to pursue a summary process action in the courts to have you removed from the property. At that point, you'll have an opportunity to defend against the eviction. In the meantime, now that you know your landlord's state of mind on the matter, you should take precautions not to give him any other reason to pursue your removal from the unit (i.e. nonpayment of rent, etc.).
This "answer" is for information purposes only and is not intended as legal advice or to create an attorney-client relationship.
The landlord can certainly mail you something. however, you don't necessarily have to leave. Entering and exiting your apartment is a normal use. The question revolves around the reasonableness of your use, not around the claimed interference to others.
Do you want accurate, personalized, legal advice that you can rely on? You will have to hire an attorney, not ask on Avvo. I am not your attorney and am not creating an attorney-client relationship by this post. I am therefore giving only general advice. This advice may not apply to you or your situation; may not take account of all possibilities, and may not match the advice I would give to a client. DO NOT rely on this advice or any other advice on Avvo to make your legal decisions. If you want an answer to a legal question you should retain an attorney who is licensed in your state.