In Thunderbird, when I'm composing a mail, I would like some visual feedback regarding who I'm sending email to, in broad terms. A typical use case is to clearly distinguish whether my mail is being sent only inside my organization or not.

Here's an example user experience that would provide what I'm after. I'm not hung on the specifics, this is just one possible way among many. First, I configure a list of email regexes and associated colors, like this:

bob@example.com        Green
.*@yoyodyne.com        Blue
.*                     BrightRed

Then, in the compose window:

  • If I'm sending a mail to Bob only, a green bar appears beneath the recipient list.
  • If all recipients of the mail (To:, Cc: and Bcc:) are in Yoyodyne, a blue bar appears beneath the recipient list.
  • If Bob is a recipient and all the other recipients are in Yoyodyne, this becomes a blue/green hatching.
  • If at least one of the recipients is neither Bob nor in Yoyodyne, then the colored bar includes some bright red (or even nicer for this case, is a uniform bright red).

Coloring the recipient names in the recipient tables wouldn't be enough, because the list of recipients scrolls, so at a point in time while I'm writing the mail I might only have internal recipients shown even though the mail would be sent externally.

Another example of a suitable design would be to configure names, for example:

bob@example.com        Bob
.*@yoyodyne.com        Yoyo
.*                     BIG SCARY WORLD

and then the title bar of the Compose window changes from something like Write: Re: fuzzy widgets to

  • Write: [Bob]: Re: fuzzy widgets for a mail to Bob
  • Write [Yoyo]: Re: fuzzy widgets for a Yoyodyne internal mail
  • Write: [BIG SCARY WORLD]: Re: fuzzy widgets for a mail sent to other people

Is there such a thing (presumably as an extension) out there somewhere? If not, what should I be looking for to program such a thing — can something be done with an extension, and where would I find the relevant documentation?

I've never seen a feature that in an email client. If you know of one, preferably one that runs on Linux, please mention this in a comment — but this question is about Thunderbird.

  • See Highlight External Addresses, and (optionally, or additionally) External Email Alert. Please note that as the status of Highlight External Addresses is "preliminarily reviewed by Mozilla", it may not appear via search in Tbird Tools (Alt + T) > Add-ons. – vWil Nov 30 '13 at 16:41

There is a Thunderbird extension that changes the border color based on the SENDING address (BorderColors GT 1.0.6). You are looking to change color based on recipients' addresses, but I wonder if the programmer of this extension could be helpful in any way.

Link to BorderColors

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