Probably was sent certified mail, express delivery and says something like:
Dear Mr. Tenant,
The law firm of Dewey, Cheatum, & Howe has been retained to represent the real estate empire of your landlords: the Sibley family. Our clients provided me with a copy of the written notice you previously received. This written notice specifies you must vacate their property at One VeryNice Place, Pleasantville, AM on or before March 15, 2021.
If you have any questions or need more information about this matter, then you should respond in writing. Please direct all such correspondence to my attention at the email or address provided above on this very official looking, intimidating, heavy weight, bonded paper letterhead stationary.
Thank you for your attention to this matter.
Cordially,
Hector Cheatum, III, Esq.
Not being a lawyer, that's what I imagine they would think is okay to send at this stage.