I check my email in multiple places, so I need my email to remain on my server until I delete it. I installed thunderbird and didn't notice the "leave mail on server for at most 14 days" check box. Tbird downloaded and erased all of my email except for the last 14 days. Is there a way to upload that email back to the server?
I doubt if you will be able to upload these emails back unless you have them backed up. Contact your web host & ask them to sync them from backups.
Why not use IMAP and that will sync everything for you? That should enable the online server to have the same emails as those in your email client....just be wary of any space limits with your online account.
Check if your mail server supports IMAP and if it does you can switch your account from POP3 to IMAP and then upload all your mesages back to the folder on your server. And you can also switch all other accounts on other computers to IMAP and then you will be checking your email right on the server. And its not going to go anywhere from the server. Its going to have a copy on your local computer but it will also leave a copy on your server. Thats how gmail works and thats how yahoo works.