I just tried forwarding an email from macOS Mail on the last email in a long threaded discussion and it opened the correct email. If you see my reply right above yours, it looks like you are NOT able to change the email subject if you want the reference link to work.
I also tested from iOS from another email thread and this also worked… as long as you change nothing about the email before sending it.
I tested going back and forth between platforms and also archiving the emails after forwarding to OF and everything is working.
Since you cannot change the subject of the email you are sending (I am assuming adding text in the forwarded email body wouldn’t cause any issues), I suppose the workflow is that you need to make the name of the task that shows up in your inbox something more meaningful. This is certainly not perfect and requires more steps than I would like, but it is a usable work around to be able to get back to emails while cleaning up your inbox.
Personally, I would like to be able to not only change the subject before sending, but be able to add the task to a project, tag it, and set a defer/due date that OF can parse on receiving. That would make my life WAY easier.
This, at least, is better than what we had before.
Answered an email about this today: posting a lightly-edited version of that response here in case it’s helpful to other folks in the future.
In addition to not changing the subject line when forwarding the email, the most important thing someone can do to increase the chances of having the link work is to avoid editing the <> characters around the link - or anything between them - in OmniFocus. Even if the content between those characters doesn’t appear to be part of the link.
Those characters let iOS know where the important data in the link begins and ends; removing or editing anything between them can corrupt the link. For more details, keep reading.
The links originate in the app which forwards the message, are transcribed by the server, and then handed off to and interpreted again by the mail reader app on the device where they’re used.
Problems at any point in that process - before the forwarded message reaches the server, even - can produce a link that doesn’t work.
It’s a multi-step process which looks like the following:
The app that originally forwards the message writes a References header into the message being sent to Mail Drop.
The Mail Drop server reads that header in and transcribes it into the note field of the OmniFocus action. To make later steps in the process more likely to succeed, it wraps the message link in those < > characters mentioned previously.
When the note field for that action is opened up, Mac and iOS handle things somewhat differently. On Mac, OmniFocus notes are rich text: this means that the start and end points of the link are marked and the actual link data is preserved behind the scenes. On iOS devices, OmniFocus provides its notes as plain text—which means it’s up to the operating system to scan that text and guess where links begin or end. That is why those <> characters are important cues to preserve.
When a link is clicked or tapped, the message ID gets passed off to the default mail app on that device. (Possibly including the browser, if webmail is in use. On a Mac, you can set the default email reader in the Mail app’s “General” preferences.)
If you’re seeing the linked email message in your email reader but it’s not opening up automatically, the next step to diagnosing the problem would be to compare the Message-Id header on that message (in the reader app) with the link provided by Mail Drop.
Some encoding does take place when turning that id into a link, but it should be recognizably similar.
Another “may help folks in the future” item: the References header used to create the links is added when a message is forwarded, but mail clients typically do not add it to messages which are Sent. If a new message is sent directly to your Mail Drop address, it will not have one of these links in the note field.
This lets you add tasks directly to your OmniFocus database without also creating extra content/noise in the note field of those actions.
