I also wrote my own system for pretty much this exact purpose, although instead of emailing me reminders, it just lets me record notes on a map and then I skim that map whenever I am planning a trip to a given country or region.

It's online (but registration is invite only, let me know if you want to play with it) over at https://scoutmaps.io/ – at one point I was going to attempt to commercialize it, but eventually decided I liked it better as hobby software so that I can write features that better serve my current userbase of one :)