I am assuming that you are using the same e-mail address in Outlook webmail and in Thunderbird. You have not used Thunderbird for awhile, so the issue probably is that either you have changed your e-mail password since you last used Thunderbird, so Thunderbird is trying to login with an old password, or you need to set up an App password for Thunderbird. Either way, you should set up an App password, since Outlook requires it these days. First I recommend making sure you have a second copy of those messages in Thunderbird, to make sure you do not lose them. Right-click "Local Folders" and select "New Folder". Name it whatever you want. Then in the folder where those messages are, click on one and then press Ctrl-A to select them all. Now right-click on those selected messages and point to "Copy To". Follow the additional pop-up menus to point to "Local Folders", then the new folder you created, and click on it. Now make sure that the messages are now in both locations. Now to set up the App password for your Outlook Mail account. Go to [https://account.microsoft.com/ https://account.microsoft.com/] and log into your Hotmail/Outlook/Microsoft Account. Go to "Security & privacy", then "More security settings" under "Account security". Go down to "Two-step verification" to set your App passwords. Once you have set an App password for use with Thunderbird, copy that password and paste it somewhere temporarily (like a text file or Word file), for safe-keeping. Then you need to tell Thunderbird to forget the password it has saved for that e-mail account, so Thunderbird will ask for the new password. Thunderbird menu: Tools: Options: Security: Passwords tab Click on the "Saved Passwords" button. If you have only the one e-mail address set up in Thunderbird, then click the "Remove All" button at the bottom left. If you have more than one e-mail address/account setup in Thunderbird, then you need to just remove the "imap" or "mailbox" and the "smtp" lines for that particular e-mail address. Then click the "Close" button, and then the "OK" button on the Options window. Now when you tell Thunderbird to check your e-mail, it will ask you for the password. Paste in the App password you set. It will also ask you for that password the first time you send an e-mail. Now you should be able to copy the messages in question to the IMAP folder for your Outlook e-mail message.