Eric and Randy's excellent contributions notwithstanding, I think the problem is even simpler from a user's perspective: 4) "Given my grandson's email address, send this email to him privately." Even if the PGP key format soon expands to include attributes of many types, it's clear that all human interaction is based on correspondence of some kind

nope. i, and many others, use pgp to encrypt files on disk should one's system be cracked, a laptop be lost, ...

SRVs are the way to go, if they enable the rapid adoption and scaling of (1) and (2) and make (4) easier for Grandma.

to use srv you have to know the domain. you have just recursed and made no progress. randy