Systems like Cyberboard and Vassal (which started as Virtual Advanced Squad Leader) were created specifically to allow playing board wargames via email. But to keep them generic enough to work with any game (and to avoid lawsuits), they do not implement the rules. They let you set up and save the state of the game, log dice rolls, playback opponent actions, etc., but it's up to you to remember and enforce the rules.

Combat Mission was a PC game similar in feel to Squad Leader. You check status of units, issue orders, then the game plays out a short burst of activity in real time before it halts for you to review and issue new orders. The original is old now, but dirt-cheap on GOG. (There are sequels, but they have DRM.)

You might like browsing places like Matrix Games, Slitherine, or HPS Simulations. They have tons of wargames, some of which have the boardgame or miniatures feel. Some computer games go so overboard on details or micromanagement as to have a totally different feel from boardgames, but others are close enough.

It's older, and not a direct port of the rules, but Combat Mission was a rad game that scratched that same itch. (There's a whole series now, but I only ever played the first.)

The Close Combat series is similar in that it attempts to be a realistic depiction of WWII company level engagements. CC is real-time though, not turn-based, although I see that as an advantage.

All my old Squad Leader buddies live on the other side of the planet (I emigrated), so turn based is essential to me.