Sorry, I just noticed your question, @thirstylizard. Here is the latest integration: Fmail now email supports standard RFC-5322 email message ID. It will be built into Hook 3.4.3. For questions about the new integration, please reply on that page (Fmail now email supports standard RFC-5322 email message ID).
In short, it’s up to you which script to use, though we feel the latest ones are better because the links it generates are interchangeable with all other email apps that use standard IDs.
FWIW, in a support ticket with Fastmail they just mentioned that you can use the browser URL for an email (I.e. you can rely on it being stable) but the conversation wasn’t about if it’s also standards compliant.
I’m trying to get them to add URLs to their iOS app.
Support have said that the URL does not contain the standard message ID
that’s a drag. Not very interoperable and furthers lockin to their product.
On a related note Mimestream expects to become link-friendly.
I don’t think it’s intentional — their values seem aligned with interoperability. I think it just wasn’t on their radar. Contributing to Open Source Technology for Email | Fastmail