(Solved, See “Added”, below)
I am far to new to Hook to have an informed opinion as to this being a bug or not, so I am posting it here in the discussion pages. I just tried looking at a Mail message in Apple Mail, and copying the link.
Then I went to a main TaskPaper document and added that link just to test to see if Hook also works with email. Weirdly, it added the link but with a Postbox mail client icon. I had Postbox many years ago but have long since stopped using it. When I double clicked on the link, sure enough, my system started to open an old version of Postbox I still had on my drive.
It does not make sense that copying the link of a mail document as in, Apple Mail, should link to anything but that application. Apple Mail is currently set as my system default email application. It puzzles me that Hook was not able to understand this, or that Hook sought out a very old email platform such as Postbox to open the linked Mail message in.
Copying a Mail message link and having Hook decide it is a Postbox trigger, makes no sense. I think it is fair to report this as a possible bug.
ADDED: Oops. It appears that there was a preference setting that came as a default setting to Postbox. I don’t know why it would choose Postbox as it has been some years since I used it or upgraded it. Changing it to Mail fixed the problem so, not a bug after all.