Is there a way to upload that e-mail back to the server? I examine my email in multiple places, so I need my e-mail to stay on my server until I erase it. I installed thunderbird and didn't see the "leave mail on server for at most 14 days" check box. Tbird downloaded and erased all my email except for the last 14 days.
Can you check if your mail server supports IMAP, and if it does you could switch your account from POP3 to IMAP? If it can then upload all your messages back to the folder on your server. And you could additionally 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. It's going to have a copy on your regional computer. However, it will additionally leave a copy on your server. That's how GMail works, and that's how Yahoo works.
Use IMAP instead of POP on all your email clients.