Hook links to emails are not bound to any particular email client.
Any application which is assigned a scheme of “email” in the script editor can handle links to emails (ie. links formed hook://email/<message-id>. If you have more than one compatible email client installed, the Favorite Apps tab in the settings window allows you to select which one to use with Hook.
Whichever application you select, you can copy links and access links from emails in any client, but only your “favorite” email client will open linked emails.