I’ve been playing my Correspondence Jam submission Wish You Were Here with a partner for the last several months, and it’s been really interesting. In the past, I had tried some play-by-email type games, and they never really worked. Usually, interest would start to dry up a few weeks into the game, so people would slow or stop responding, and the game would fizzle out after two or three months at best. We’ve kept this going, perhaps because it’s a smaller conversation between two people (who otherwise see each other regularly).
Some skills from tabletop roleplaying carry over into epistolary RPG play, and some don’t. Obviously, speaking in a silly voice doesn’t help. But being able to create a complex, interesting character and convey them through words and actions does apply. Another thing from tabletop RPGs that still helps is being a good, supportive partner: listening and building on what they say, finding ways to boost the other player, offering them openings to do something cool, thinking about how to steer the story in interesting directions, etc.
So it feels like roleplaying, but not like roleplaying the same way you do at the table. A bit also like fiction writing, for obvious reasons.
The pauses between letters in other games was the death of the game (see above). For this game, though, the long pauses between letters has been an important part of the experience. It’s a game about loss, mourning and death, so the pauses between letters helps make that loss seem a bit more real, and makes the letters more significant when they do actually arrive.
But the structure is also important in pacing of the game: You can’t have the same back-and-forth type conversations in letters or emails as you can in a face to face game. Trying to hold a line by line conversation using letters will take weeks to get through even a short exchange. Ideally, instead, each player can create their own chunk of content, such as a single letter covering an event, and then other player(s) can make their own chunks in response. Most tabletop RPGs are made to have fine grained back-and-forth, especially in action and combat, and that works very poorly in an epistolary format.
One last point: even if you are writing letters or emails back and forth, it’s a good idea to have an out-of-character 21st century method of communicating about the game. A shared text message thread, or a shared Google Doc of notes, or a discord chat or something. You can use that to let the other players know if the game will be delayed, or to clarify rules issues, or just to check in that your correspondent(s) are enjoying the direction the plot is taking.