You refer to one option that would recreate the inbox and another option to use the old one. Usually, keeping the old profile is best as that is exactly what is working today. My suggestion is to try your desired solution now, prior to the reinstall. If you think removing the inbox might work, then rename the folder (with TB not running) and restart TB to see if it reconstructs your inbox satisfactorily.

David,

I did as you suggested and deleted the existing inbox and inbox.msf files. (rather, as is what I think is best practice, I renamed them OLDInbox and OLDInbox.msf respectively)

Thunderbird does successfully re-create the Inbox. However, any mail from other people still presents the "A unique identity matching the From address was not found" error. This includes senders who are definitely in my address book.

Meanwhile, a test email sent from one of my accounts to another can be opened without creating the error.

Do you have any addons that may be causing this? A quick web search indicated this may be a known bug.

The problem persists even when booting Thunderbird in Safe-Mode. The known bug (dated four years ago and still affecting people today) was originally for OSX, which I assume is similar enough to Linux to affect users like myself. That bug report is where I found the ctrl-o workaround. I've been hoping for a bug fix for over a year now. As a side note, opening a message in a tab isn't affected. Only opening messages in new windows.

The problem persists even when booting Thunderbird in Safe-Mode. The known bug (dated four years ago and still affecting people today) was originally for OSX, which I assume is similar enough to Linux to affect users like myself.

I doubt it as it is a keyboard issue. Are you using Apple keyboards? On OSX? That bug has only ever been reported by OSX users pressing return on the numeric keyboard made by apple. https://bugzilla.mozilla.org/show_bug.cgi?id=1508153

Are you using the unified inbox? in which case deleting it and allowing it to recreate might help. Close Thunderbird. Open the profile folder and locate the smart folders in the mail folder/directory and delete them. Restart Thunderbird.

Otherwise I would be more inclined to think there was an issue with identities. ie mail to Bully@beef.com that is collected by say a gmail account bully@gmail.com will not have a unique identity to reply with unless you configure one.

Matt;

I am on Ubuntu using a standard USB PC keyboard with the most common English/qwerty layout. Which means my keyboard sends scancode 40 when using the main enter key adjacent to the qwerty layout. The key associated with the number pad sends scan code 88. 

In most contexts, the two keys are identical in function. On my system, using either of the two return/keys creates the same error. CTRL-o or right click-open in new tab does not create the error. What struck me about that OSX bug was that it affected only use of the main return key. Using the numberpad return key (on keyboards that had a numpad) didn't seem to trigger the error.

I have already attempted the deletion of the inbox and allowing TB to recreate it. But that doesn't work. I have also tried deleting and recreating profiles.ini. I thought, as you suggest, that the issue may lie in the identity profile itself. Not in mapping case variations as in your example, but something deeper in the way TB interacts with the profile framework itself. For what it's worth, I have tried creating additional profiles and switching to them as a test. This also didn't work.

So, my current desire is to back up just the content of my current profile, but not the profile and inbox itself. Export everything to .json and .csv files, include user created folders and sorting rules, then do a full uninstall and purge of TB, followed by a fresh install and then import my data into the auto created profile. (calendar data is all in the cloud already)