Reply to sent mail go to a different folder

Hi,

In my one account in Thunderbird, I still have the old POP folders, that was a different account. I also have an IMAP account, both have the same address. The pop one does not work but the folders are there, I use them to look old mails up, so far so good. When I sent out an email with my IMAP account, the sent mail goes into the sent folder of the IMAP account, so far so good. BUT when I click on that sent mail and click on reply, it replies to the same address that what it already was sent to, now here comes the problem. THAT copy of the mail, I called the NEW sent, does not go into my sent folder. It goes directly into the sent folder of my old pop account. Even though everything on the IMAP account is standard, re-sent mails are not placed in the IMAP account sent folder. What am I doing wrong here, I know when writing the mail is that I can choose the send address and if I change that it goes in the IMAP sent folder but why is it standard on my old POP account, and why not on the IMAP account even though the IMAP is the standard?