The question https://support.mozilla.org/en-US/questions/1057940 might be about this same problem, but it's hard to tell since the reporter never replied. In my case, this is definitely not about the "Local Folders" account.

In my case, this is definitely not about the "Local Folders" account.

So are you saying you do have a valid 'Local Folders' account? The "server7.invalid" may be an orphan account resulting from the Postbox profile import.

In my case, this is definitely not about the "Local Folders" account.

So are you saying you do have a valid 'Local Folders' account?

Correct.

The "server7.invalid" may be an orphan account resulting from the Postbox profile import.

Yes, that seems likely to be true. So how do I get rid of it?

You can try the following.

First backup the prefs.js located inside your profile folder. See https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data for more information on how to find the profile.

Then open the Config Editor. https://support.mozilla.org/en-US/kb/config-editor

Filter for the invalid account by typing "server7" (without the quotes) in the field showing 'Search preference name'.

What do you get? Does the result match any existing account?

OK. In the config editor there are lots of matches for "server7", all beginning with mail.server.server7:

mail.server.server7.ageLimit 30 mail.server.server7.applyToFlaggedMessages false mail.server.server7.canDelete true mail.server.server7.cleanupBodies false mail.server.server7.daysToKeepBodies 30 mail.server.server7.daysToKeepHdrs 30 mail.server.server7.directory /Users/bhaller/Library/Thunderbird/Profiles/iyt5uh9d.default-release/Mail/Imported Mail mail.server.server7.directory-rel [ProfD]Mail/Imported Mail mail.server.server7.downloadByDate false mail.server.server7.downloadUnreadOnly false mail.server.server7.hostname server7.invalid mail.server.server7.index_msgs true mail.server.server7.lastFilterTime 29397136 mail.server.server7.name server7.invalid mail.server.server7.nextFilterTime 29397146 mail.server.server7.numHdrsToKeep 2000 mail.server.server7.sortkey 5 mail.server.server7.spamActionTargetAccount mailbox://nobody@server7.invalid mail.server.server7.spamActionTargetFolder mailbox://nobody@Imported%20Mail/Junk mail.server.server7.spamLevel 0 mail.server.server7.storeContractID @mozilla.org/msgstore/berkeleystore;1 mail.server.server7.type none mail.server.server7.userName nobody mail.server.server7.whiteListAbURI moz-abosxdirectory:/// moz-abmdbdirectory://abook.mab

What next?

OK. In the config editor there are lots of matches for "server7", all beginning with mail.server.server7:

mail.server.server7.ageLimit 30 mail.server.server7.applyToFlaggedMessages false mail.server.server7.canDelete true mail.server.server7.cleanupBodies false mail.server.server7.daysToKeepBodies 30 mail.server.server7.daysToKeepHdrs 30 mail.server.server7.directory /Users/bhaller/Library/Thunderbird/Profiles/iyt5uh9d.default-release/Mail/Imported Mail mail.server.server7.directory-rel [ProfD]Mail/Imported Mail mail.server.server7.downloadByDate false mail.server.server7.downloadUnreadOnly false mail.server.server7.hostname server7.invalid mail.server.server7.index_msgs true mail.server.server7.lastFilterTime 29397136 mail.server.server7.name server7.invalid mail.server.server7.nextFilterTime 29397146 mail.server.server7.numHdrsToKeep 2000 mail.server.server7.sortkey 5 mail.server.server7.spamActionTargetAccount mailbox://nobody@server7.invalid mail.server.server7.spamActionTargetFolder mailbox://nobody@Imported%20Mail/Junk mail.server.server7.spamLevel 0 mail.server.server7.storeContractID @mozilla.org/msgstore/berkeleystore;1 mail.server.server7.type none mail.server.server7.userName nobody mail.server.server7.whiteListAbURI moz-abosxdirectory:/// moz-abmdbdirectory://abook.mab

What next?

mail.server.server7.name server7.invalid

That looks like your orphan account. Make sure you do have a backup of the prefs.js file. Then delete all the entries starting with "mail.server.server7" by clicking the trash icon following the entry.

Restart Thunderbird. The invalid account should be gone.

The "server7.invalid" account is gone, and Thunderbird relaunched and seems to be working normally so far. Whew! Thanks for your help! (It seems like the root bug here is maybe that that account was marked as "special" somewhere, and thus I was not allowed to delete it in the UI and there was no way to override that block in the UI. It would have been great if there was a way to do this without the hand-editing of the internal prefs list, which was a bit scary! Of course it would be nice if the "import" process didn't create the spurious account in the first place, but I can see that being harder since you don't really know what's spurious and what's not.)