ultimately in the OP, does everyone hosting email and using Lets Encrypt on Plesk have the same issue with iPhone Emails? or am I just special
FWIW We use Let's Encrypt *Wildcard Certificates on both the host.domain.com and lots of individual domains. In our case, they all share the same IP address. We don't use Certbot and we don't use the Plesk Let's Encrypt extension because, there's a bit more work to be done on it yet, for use with *Wildcard Certificates, but non *Wildcard Certificates are fine using the Plesk extension. We use
THIS provider so it's more of a manual process, but it's simple, easy and works everytime without any problems for us. *Wildcard Certificates renewals need careful organising though, as they are also still a manual process, not automatic.
Perhaps most relevant to your post, is that we don't have any issues with
any mail clients that are being used, including many iPhone users. That maybe down to how mail is setup within Plesk in terms of Server Settings and Mail Settings and the various choices that are available within there though, but certificates have never been an issue with mail for us.
We're guessing, but are reasonably sure it's correct, that if, you have a separate IP address for each domain (including a separate IP address for host.domain.com) then the plus side is that the whole thing becomes very easy by comparison. The minus side being, that there's a lot more (repetitive) initial setup work to do, especially if you have many domains...Can't comment more than that, as we're currently moving everything over to a new Ubuntu 18.04.1 / Plesk 17.8.11 server setup, but we're not anticipating any Plesk mail problems on that either, to be fair