I would advise that it is highly risky having one simple text file containing 31,000+ messages. Advise would be to create some suitably named folders and move batches of emails as appropriate to those folders for storage, so the Inbox can be used as an Inbox for unread or new mails. It is less risky to have your emails organised into various folders because they will be not in one simple text document.

Selecting all emails and using right click > Mark > as Read Would put you in the same position, but now not knowing what was still unread.

Moving batches of old mail - which are more likely to have been read - to other folders and then marking them all in one go as read would deal with a significant amount fairly quickly.

re : I do have a fresh backup (a few hours). Is that a backup of just the Inbox mbox file or entire copy of Thunderbird profile ?

Are you talking about an IMAP account or a Pop account ?

If IMAP: The problem is the server has everything marked as not read. If you replaced the Inbox mbox file then when you restart Thunderbird, the first thing it does is connect to server and update Thunderbird, so all those emails would get marked as unread again. So the best option would be to put the backup of 'Inbox' mbox file into the 'Local Folders' mail account. Then at least this would allow you to rediscover which emails were truelly unread. So, then you could mark all Imap Inbox as 'read' in one go and future received emails would be unread. In the meantime, the emails in 'Local Folders' would tell you what is 'unread', so you could read them from that location and manually alter the emails in the IMAP Inbox folder as required to be 'unread' if necessary.

If POP: You could just replace the 'Inbox' mbox file if that is the only part effected.

Menu app icon > Help > More Troubleshooting Information Under 'Application Basics' section - Profile Folder - click on 'Open Folder' A new window opens showing contents of profile Exit Thunderbird now

click on 'Mail' folder

If POP: click on 'pop mail account name' folder The 'Inbox' mbox file has no extension - it contains emails. Move the 'Inbox' mbox file to the desktop as a temporary backup measure - you can delete later if all goes ok. Copy the backup 'Inbox' mbox file and paste it into the pop account so it replaces the one you moved to desktop. Delete the 'Inbox.msf' index file - a new one gets recreated when you restart.

If IMAP: Click on 'Local Folders' folder If current Inbox has no emails, then delete it. Otherwise rename eg:'Local Inbox' Copy the backup 'Inbox' mbox file and paste it into the Local Folders mail account. Delete the 'Inbox.msf' index file - a new one gets recreated when you restart.

Start Thunderbird.

Full archive of everything, POP. I'll try it. Thanks.