Do you really need 70k messages in your Inbox? The flags appear to me a warning about nearing a bandwidth limit imposed by the mail provider. Repetitive downloading might be caused by a security app, if you have one scanning the TB profile folder.

Thanks for your response. I would love to have fewer than 70K emails in my inbox. However, I can't find any email client (including Thunderbird) that can efficiently move 70,000 emails into another folder. The most I can seem to move is ~100 emails at a time (i.e., repeat manually 700 times) or else the operation simply fails. Any suggestions?

Try copying messages to Local Folders instead of another IMAP folder, or access the account from webmail, move the messages to another folder, then subscribe to that folder in TB.

Thanks for the suggestions. I was hoping to avoid local folders (copying 70K messages over the wire) and do the copying entirely on the mail server, but I suppose it's a last resort!

Webmail doesn't help -- it displays only 35 messages at a time, so even selecting a few thousand messages (let alone moving them to another folder) is incredibly cumbersome.

This is my first experience trying to clean up large numbers of remote messages (for my spouse), and I'm kind of appalled at how poorly all of the tools scale! As a software engineer, I don't understand why moving 10 messages should work while moving 10,000 messages fails, if the messages are just moved sequentially....

I wouldn't recommend downloading the messages to Local Folders and then trying to upload them to an IMAP folder; just keep them in Local Folders. If the TB Inbox has already synced with the server, the messages are on the local computer, so copying them all at once should work, as long as there are no security apps interfering. Once the copy is confirmed, delete from the Inbox.

If this were a gmail account, the takeout option would allow the download of the folder as a single mbox file, which can be easily imported to TB.

Yes, messages can move to another folder in the same account - no need to use Local Folders.

A more reliable method of moving messages may be a filter. Create a filter to move every message old than 6 months (for example) to folder X. Then manually run that filter against your Inbox.

sfhowes: Thanks for all your suggestions. I prefer the emails to live in an IMAP folder so I can access them from multiple devices (laptop, phone, tablet).

Wayne: Thanks very much for the filter suggestion. I'll try that.

! As a software engineer, I don't understand why moving 10 messages should work while moving 10,000 messages fails, if the messages are just moved sequentially....

The connection times out and a bug in Thunderbird does not recover...

Matt - thanks for the insight!

Update for anyone interested:

• Thunderbird message filters didn't work. I defined a filter to move all messages between November 1 and December 31, 2020 to another folder, and ran the filter manually. The filter log reports that a few dozen emails (out of thousands) were moved, but in reality, no messages moved.
• I tried using MS Outlook (Mac desktop client) to select and drag 30,000 messages from the inbox to the other IMAP folder. Eight hours later, Outlook reports that the move succeeded and folder syncing is complete; but on the server, I see only about half the messages moved (a random subset). Thunderbird, meanwhile, still thinks that none of the messages moved and believes that its folders are all synced & up to date.

Sigh. Modern email clients are clearly not designed to manipulate large numbers of messages.

Thanks everybody for the advice. I do appreciate everyone's attempts to help.