Retrieving different email accounts to one machine and replying as those distinct accounts

I have a number of email accounts. So far, what I have been doing is
to run Eudora in my old Mac (OS9) and pop3 all the emails from those
accounts. In principle it works fine since Eudora allows me to reply using
the return email for each account. But, I would like to do things a bit
better in my OSX box. What I have in mind is to be able to retrieve those
emails, apply spamassassin/whatever to them, place them in maildir folders
(each email account having its own folders or all together, but all in my
home directory), and then be able to check my email using imap-ssl but
still be able to reply to each of them as the right user@domain.com for
each account. Can I do that? FYI, panther does come with postfix.