@jpaul it depends on the scenario. The best move for expert users doesn't change, which is to use an ESP like danwin or onionmail w/a PGP-capable MUA like mutt or Thunderbird. For novice users it's non-trivial, but generally #tutanota, #hushmail, or #countermail.
@jpaul Losing #Protonmail is a big loss for expert-to-novice comms b/c an expert can get a novice's PM pubkey & reach them securely w/out having a PM acct themselves. Apart from PM only Hushmail can do that AFAIK & HM is no longer gratis.