Dear Holmes was review by the New York Times three years ago (June 2020) back during the pandemic. The program has gone from strength to strength with over 40 cases now produced over the last five years.
Here's the NY Times review in full:
DEAR HOLMES is an epistolary mystery experience set around Arthur Conan Doyle’s immortal Sherlock Holmes character, now in the American public domain and available for new cases. You sign up for “Dear Holmes” online, but the rest of the game is purely analog in the form of paper letters, sent to you the Victorian way — by post.
The premise is simple: A series of missives addressed to the Great Detective arrive in your mailbox every week or so, asking for help with an emerging case. The sender could be a Conan Doyle regular like Inspector Lestrade or Tobias Gregson, or someone unfamiliar. The first dispatch sets up the scenario by outlining the known details of a crime or conundrum, with observations about suspects and events.
Over the next month, new letters arrive with additional clues. By scrutinizing the dispatches, you get the chance to solve the mystery yourself — before Sherlock finally replies with his own deductions in the concluding letter: As in Conan Doyle’s original stories, Mr. Holmes gets the last word in this format, too.
“Dear Holmes” was created by Michael Sitver, a Sherlock fan. Sitver’s company, Letterjoy (which mails re-creations of historic letters from notables like Clara Barton and George S. Patton to subscribers), teamed up with MX Publishing (a business devoted to new Sherlock Holmes-themed content) to start crafting the mysteries-by-mail in 2018.
The Holmes letters, usually running several pages, arrive in your mailbox on thick, cream-colored paper with matching envelopes. Depending on the fictional sender, the contents may be composed in a typeface that resembles the fluid scratching of a metal fountain-pen nib or the worn serif font of a turn-of-the-century typewriter.
For those who remember the glory days of personal correspondence by snail mail, the tactile sensation of opening and poring over a long letter may put your mind right into the unfolding story. It’s not quite “Method reading,” but it definitely sets the mood. More than a dozen “Dear Holmes” mysteries have been produced so far and, even if Dr. Watson’s perspective is missing, they admirably echo Sir Arthur’s style and tone.
Subscriptions to “Dear Holmes” start at about $50 for three months of Sherlock’s mail, and a new mystery starts each month. The timing of each new installment depends on the postal system, which adds an additional touch of authenticity and anticipation to the whole experience. It does offer the hope of something in the mailbox besides bills and junk mail. (While it may be a wobbly time for paper-based deliveries because of coronavirus concerns, the United States Postal Service cites several health agencies that the Covid-19 virus cannot be spread by mail. Still, if you’re worried, let the letters sit for a few days after delivery.)