I don’t think email is a “platform”. The real platform is the mailing list that Trump’s campaign has built (or purchased). And if their infrastructure provider starts blocking sent emails because they break acceptable use policy, they need to switch to another infrastructure provider. Basically facing the same situation that Parler did.
Unlike Parler though there is no single point of failure – Trump can self-host his email, and even his mailing list management app (eg Mailchimp alternative). Then he only needs connection to the Internet, which hosting firms all over the world could offer.
Why would he self-host email, instead of switching to something like ProtonMail (which, as far as I’m aware, does not suspend users outside of illegality)?
ProtonMail is good for one-to-one mail, but the article is about Trump using bulk email for fundraising (eg the 400+ email campaigns he sent in 30 days following the last election). Bulk mail is a different service, with providers like Mailchimp, but in case those companies blocked (deplatformed) him then he could self-host without fear of getting blocked again. In fact Trump did used Campaign Monitor (not Mailchimp), and they did block him.