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An answer on stackoverflow tells me that there is a [draft] standard URI schema for referring to parts of an email message by their Content-ID and entire email messages by their Message-ID. Another stackoverflow answer shows how a cid: URL might be used in an HTML email to reference an attached image.

What clients support cid: URIs for referencing attached images in an html email? (I expect most)

What clients support mid: URIs for referencing images in another email in an html email? (I expect few to none)

What clients support mid: URIs for referencing whole other emails, either within new emails or from outside sources? (I expect few)

Of these, is this support optional or non-default in any of them?

Community
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Sparr
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2 Answers2

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Thunderbird

Prior to version 78, Thunderbird had support for mid: URLs via the Thunderlink addon, version 1.2.11 and later. However, with the transition away from Legacy WebExtensions in Thunderbird 78 and later, Thunderlink ceased to function. Fortunately, Thunderbird added native mid: URL scheme support in version 91, completing a feature request dating from 2004. This works both for opening external mid: links via command line, e.g.:

thunderbird mid:123456.789012@example.com

as well as opening internal mid: links in emails inside Thunderbird.

Evolution

GNOME Evolution has supported mid: URLs since version 3.39.1 and later, completing a feature request from 2017.

Apple Mail (sort of)

Apple Mail added message: URLs in 2007, which appears to work in a similar manner just with a non-standard URL scheme. (Thunderlink also had support for this non-standard scheme.)

cid: URLs

Thunderbird did not add cid: URL scheme support. Neither did Evolution. As far as I know, no email client supports the cid: scheme as its usefulness seems to be not as compelling for most users, although it could be used to e.g. link to message attachments.

Related:

Nathaniel M. Beaver
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(Not a full answer, but maybe helpful)

For the mid + Thunderbird part of your question (however, please note that I didn’t test these add-ons yet, so I’m not sure if they really use mid URIs or some proprietary protocol):

The description of the Thunderbird add-on ThunderLink says (bold by me):

ThunderLink lets you link to email messages in Thunderbird.
[…]
ThunderLinks are based on the unique message ID generated when an email is sent.

The companion Firefox add-on ThunderLinkSpotter

[…] enables the browser to recognize ThunderLinks and turn them into hyperlinks.

Another Thunderbird add-on is MessageID-Finder, which allows to open mails by right-clicking on a message-id (see a screenshot on the website). But this would only work from within Thunderbird.

unor
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