[JohnFen]

I'd recommend writing those things as physical letters on paper, but if you're really into using email for this, you might consider standing up your own server to do that. It's not as hard as you might think.

I personally can't think of an online service I'd trust to keep my emails for 20 years. I'd absolutely want them on a machine I'm running myself.

[silisili]

Yup. Definitely written letters, or at worst print out the emails.

Even if the service is around in 20 years, there's a high probability that the account will be disabled or deleted for nonuse.

[netsharc]

I wonder if Western Union can help: https://www.youtube.com/watch?v=iA1LmWWYxWY