I wish we had better words to describe encryption and the specific tradeoffs of each approach. I did not know purelymail, but knowing IMAP I had a gut feeling that things were a bit more complicated than a blanket "fully encrypted on their servers".
Sure enough, reading between the lines of their documentation they can pretty much decrypt any email on an account by just using the password given by the client when connecting to their IMAP server. Since most clients either connect regularly to fetch emails or maintain a long-lived connection to the server, they can pretty much decrypt anything, any time. So it's back to trusting them just like it emails were stored in plain text.
I don't want to pick on this small player, I applaud their effort in pushing email forward, but I have enough with companies using encryption to handwave security concerns. A big example of that is Apple iCloud.