Reset mac mail. New question

I have tried everything I can find on this within the support community. I need to reset my mac mail. I had to buy a new mac as the old macbook 1.1 died. When I set up mail it went to my POP account and downloaded ALL my emails. For some reason I could not send emails and it seemed at the time that it would be easiest to delete the account and set up again. Unfortunately now I can only get the 'new' emails. These go back to the first time I used this mac. The POP account is set to give all the messages. I have marked all the messages as new. I have deleted everything I can find that relates to a preference. In Library>Mail and SyncServices and Application Support and Containers. Also redeleted the account from within mail. Restarted the computer as well after deleteing all this.Multiple times done this in case missing something and deleting all the new info after starting and setting up mail again. I'm goin' nuts. There must be some way to reset this and get all the mail again. Using 10.5.8 and Mail 6.6. Would appreciate any help.