Additionally to what others mentioned, Dnd Beyond [1] has a quite active play by post board on their forum. It's great to get started, but games tend to rarely go to the end of the campaign and stop after a few months. It's still a good thing as a first experience.
After that, I would recommend to find friends to play by mail. Somehow, I noticed players less easily give up the game when they're friends. Simply by talking regularly about your first games is usually enough to get other friends interested. Just don't push it on them and let them come by themselves, as it's quite a commitment and will be felt as a burden if they feel they're playing with you just to be nice to you.
Yes, play by post most certainly is still a thing. Most RPG forums (e.g. https://forum.rpg.net/) have a play by post subforum, and https://rpol.net/ is a forum dedicated to play by post.
There's also live online play e.g. on discord or roll20. Each table is different, do a search for groups on social media and ask about what their games are like.
After that, I would recommend to find friends to play by mail. Somehow, I noticed players less easily give up the game when they're friends. Simply by talking regularly about your first games is usually enough to get other friends interested. Just don't push it on them and let them come by themselves, as it's quite a commitment and will be felt as a burden if they feel they're playing with you just to be nice to you.
[1] https://www.dndbeyond.com/