I am impressed by your success at moving your thousands of messages from Agent to your Local Folders folder.

But I don't understand how you moved an account's contents from a server that only supports POP, and you did the transfer in IMAP. Or was "Agent" your email client (software), and your server (provider) an IMAP server?

If Agent only supported POP, how did it happen to have copies of your messages available?

And the big question: Why in the world would you move perfectly good messages from a secure folder on your computer INTO an IMAP-connected INBOX, when the IMAP server is only prepared to have messages in your INBOX that originated from the current server?

I'm just plain confused, and hoping to get some clarification from you.

I am impressed by your success at moving your thousands of messages from Agent to your Local Folders folder.

Instructions at https://groups.google.com/g/alt.usenet.offline-reader.forte-agent/c/6yW3XAY0ltY/m/eA1k5tbkAgAJ

But I don't understand how you moved an account's contents from a server that only supports POP, and you did the transfer in IMAP. Or was "Agent" your email client (software), and your server (provider) an IMAP server? If Agent only supported POP, how did it happen to have copies of your messages available?

Agent is the Windows-based POP email client I'm trying to move off of. POP email clients store emails on the client PC.

And the big question: Why in the world would you move perfectly good messages from a secure folder on your computer INTO an IMAP-connected INBOX ...

See the benefits of IMAP. One example: When I travel it will no longer be necessary to move the email database from the desktop to the laptop and back again.

when the IMAP server is only prepared to have messages in your INBOX that originated from the current server?

I am confused. I read the article on moving from POP to IMAP to say that messages can be moved to the IMAP server by moving them from the POP folder to the IMAP folder. Is that wrong?

And if it's right, why aren't the messages moving properly?

Which server is your IMAP server? (GMail, ...) Did you create any special folders on the IMAP server to store your old messages in? I'm not at all sure about this, but my feeling is that moving your old messages to the INBOX from your local folders is going to be less than successful. The INBOX folder is for messages that have been received by the server and relayed to your computer. If you go to the server's online browser interface and create a new folder there, that folder will be duplicated on your PC by the server. Then moving mail from a local folder to the newly created folder should succeed. Could you try that with one new folder and the transfer of a few old messages?

"messages can be moved to the IMAP server by moving them from the POP folder to the IMAP folder."

You don't have an IMAP folder, ofc. You have several folders that came with your account on the server: Inbox, Sent/Sent Mail, Drafts, Spam/Junk, and Trash. All of those are special-purpose folders.

But if you create a new IMAP folder (Label, in GMail), it's yours, and you can put whatever you want in it. And Your server host will sync to that folder and copy its contents to the server.

But with online folders/labels, there can be space limits that are strictly enforced.

If you store messages that are not of immediate importance to you in Local Folders folders, the only space limit is the size (capacity) of your drive.

Which server is your IMAP server? (GMail, ...) Did you create any special folders on the IMAP server to store your old messages in? I'm not at all sure about this, but my feeling is that moving your old messages to the INBOX from your local folders is going to be less than successful. The INBOX folder is for messages that have been received by the server and relayed to your computer. If you go to the server's online browser interface and create a new folder there, that folder will be duplicated on your PC by the server. Then moving mail from a local folder to the newly created folder should succeed. Could you try that with one new folder and the transfer of a few old messages?

Thanks for your suggestions. This account is hosted on Dreamhost. I tried your suggestions and a few others, and none of them shed any light. But on inspection, the problem messages are the ones that were sent from the account (originals or replies). When moved to the IMAP folder on Thunderbird, those sent messages get the date/time of the move. But when synced to the server they get their original date/time back.

I'm thinking that the Dreamhost server uses the date/time in the From line, but Thunderbird uses the Received field in the message header, or if it's missing (as it is in the sent messages), then the current date/time.

So a workaround might be to insert a Received field into each message's header in the mbox file using the From line's date/time.

Maybe I'll check first to see whether Gmail agrees with Dreamhost or Thunderbird.

It's not that simple. Apparently this processing is done by the server, which needs a minimum message header to parse it as a received message. Otherwise it treats it as a new message.

So a workaround might be to recognize which messages were sent by Agent and add the missing elements into their headers. I'll work on that.

Best of luck to you. You’ll need it. Did you create that Gmail folder/label, did it get copied to your main folders in TB, and are the messages that are getting their original dates restored (instead of the “moved to the server “ dates) now synced between TB and Gmail successfully? You may have to go into settings > folders on Gmail to set the created folder’s display properties.

It's not quite that complicated. Turns out that all that's missing from messages originated in Agent is a Date: field in the header. All messages are required to have a timestamp in the From message separator line, but the servers (at least Gmail and Dreamhost) won't accept that.

So the workaround is easy, if a bit brute force: In Agent's exported mbox file, use a regex editor to insert a Date: line immediately following the From separator line of each message, using the same timestamp. Some emails will already have a Date: line, so now they have two. But it doesn't matter as long as its timestamp agrees with the From separator line. Based on the few I've seen, I'm willing to believe they always do, until told otherwise.

(If the timestamps don't agree, GMail and Thunderbird both use the first Date: line. Dreamhost does it differently. But as long as they all agree it's a moot point.)

My simplest account - the one without folders - has successfully imported all 2,865 messages into INBOX. When all the others are imported, I'll post instructions separately.