As an eternal user of Thunderbird and its predecessors (going back to the email component of Netscape Navigator in, I guess, 1994), IMNSHO, the dev has produced an email client that - for my daily needs - doesn't require any further development, except perhaps as security threats emerge.
I find the result of the effort nothing short of amazing - and an absolute lifesaver in reply to my provider's decision to kill clients that don't implement the latest versions of TLS.
To be honest, I don't remember how I got Epyrus to the state I now have it, but I've been able to import all my settings, current and saved mail, address books, UI tweaks, etc. This is in contrast to the latest Thunderbird, Mailbird, and I don't remember what else, all of which choked at some point.
In particular, I was able to import my saved mails into Epyrus from my ancient Thunderbird v184.108.40.206pre and see them in my folder lists. By contrast, when I tried to read one in the newest TBird & co., the text window was blank, and I was unable to remedy this. With Epyrus, everything was perfect on the first try! Thank you athenian200.
However, as Rosanne Roseannadana said, "There's always something ..."
I submitted this to the SumatraPDF forum and was told to try my luck here.
Thanks.MS have shifted a lot of underlying feature in the name of security, I used to be able to view remote pdf and other file types via Trident (the core of Internet Explore) Recently that seems ? (unconfirmed via other older devices) to have been fully removed with the EOL of Internet explorer.
File associations such as epub and pdf were messed about with the introduction of Edge.
Mailto may have depended on one or both aspects , however cant currently load either of above to test for present, perhaps reach out to viewforum.php?f=73 (For what its worth I can use Pale Moon with SumatraPDF to browse PDFs, but have not tried xps/djvu recently)