Pop mail accounts by default usually download from server inbox to the mail account Inbox and then delete the copy on the server. It is considered Pop mail accounts do not use the server as storage, hence the auto delete. However, there is usually an option to 'Leave copy of message on server'. I do not use Outlook so cannot say where it is, but in Thunderbird it would be in the Tools > Account Settings > Server Settings for mail account. Outlook may have it in a similar location. As Thunderbird was accessing the same server using IMAP, TB sees a remote version of what is on the server. I would try exporting the Outlook emails in .pst format then convert .pst to mbox file. In Thunderbird. download and install addon tool 'ImportExporttool' * https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/ then import the mbox file emails into Thunderbird 'Local folders' mail account. Right click on 'Local folders' > ImportExportool > Import mbox Then copy them back into the imap mail account.