If the phone is IMAP, you would need to upload the messages to your IMAP account from your PC. If you set the phone to POP, then you would need to email the messages to yourself so the phone would see them.

Hello David, I'm still a beginner with Thunderbird, so I'm not sure what you mean. My email account with my email provider was full. So I downloaded about 1,000 emails to my PC and deleted them online. I was hoping I could access them again with the mobile app. But that doesn't seem to work. Sending 1,000 emails manually can't be the solution. Besides, that would fill up my account again. Too bad. Thanks for your reply. Regards, Tom

You do not say how you downloaded your mail to the desktop.

If you used something like Googles takeout, have you imported them to your desktop Thunderbird? If not did you downbload via an IMAP in Thunderbird? If so, you might want to check that you actually have any emails on your desktop to import anywhere. IMAP is syncronised so if you deleted the mail from an IMAP account in Thunderbird then synchronization will see those email also deleted in Thunderbird on the next connection to the mail server.

I've solved the problem. I have another email account with more storage space. I logged into that IMAP server with TB and then uploaded the 1,000 emails using drag and drop.

That was really easy. I wasn't familiar with TB before and was in a hurry to empty the account, which was overflowing. Now I know more!

Thanks again for the support.

Regards, Tom