How to get rid of Thunderbird error “[NONEXISTENT] Unknown Mailbox: Trash”

Thanks for your suggestions, but here's a much much quicker way of doing it - doesn't require doing anything in gmail!

ALL IN THUNDERBIRD: synchronise gmail and thunderbird folders right-click the folder with the problem, properties, synchronization, download now

create a new folder called “test” (in local folders, not an imap folder) select any number of the messages, right click and select “copy to” → “test” if you get error message “while working offline, you cannot move or copy messages that were not downloaded for offline use ...” then there's a corrupt message in your selection.

Select a smaller number of messages and repeat until you don't get the error message. Then increase until you do again. Her presto - you find the problem message.

IN GMAIL: Delete the offending message.

Done!

"Sent: do NOT store sent messages on server"

Toggle this, and sent items will be stored on the Gmail server, thus enabling you access from both desktop and iPhone.

I would suggest you set all the "Store on server" options to Checked. Google's not always correct, or up to date; although they do try their best. I've never had issues with all the settings to store data on the server in the last 4-5 years Gmail IMAP has been available.

Also, look at what you have set as your Sent folder. In the list of mailboxes, scroll down until you see something like "Gmail" or whatever you named that account. Expand the list. If you see another folder in there with a name similar to "sent", it may be that your missing emails are there. Drag and Drop your known emails to this folder, then select it again, and from the menu, choose Mailbox > Use this mailbox for > Sent. Similarly for trash, junk, and drafts. I find it also helpful when dealing with an Exchange account over IMAP since Outlook on Windows XP has a different naming convention than Apple's mail client software.