Copy Mail.App messages to Thunderbird

[artov]

I made a bad mistake: my job email server supports IMAP, so I read email
using Thunderbird (on Vista), but keep the mails at the server. I had to go to
client for a week, so I set the account for Mail.App. But, I set it to use POP, so
all the email was moved to my Mac from the server.

Is there any clean way to move the email back to the server or to the
Thunderbird on my PC? I guess I could send all the important emails to
myself, but

- how can I be sure what messages are important?
- how can I keep the days right
- how can I keep the sender right