Many mail clients support opening specific mails using a special URL scheme. Because engineers are engineers, obviously there is more than one standard here. For example, Evolution usesmid:<messageid> and macOS Mail usesmessage:<messageid>.

If the user specifies what URL scheme they prefer, then patchwork can add buttons that will open the relevant mail directly in the user's mailer (assuming they have it). It looks like this:

Clicking the "open in new window" button opens the mail in my client like magic! This makes replying to patches actually easy, which is nice for a change.

Marking as a draft as the commit messages need work and I'd like feedback from the maintainers, but this does actually work.