You should install postfix as an alternative (and easier to work with) than sendmail.
You would want to run fetchmail as either a service or a cron job - this will download your mail from your ISP. The file you edit is $HOME/.fetchmailrc
You should also investigate spamassasin. I have not used it, but this is what you want to filter out your junk mail. I believe it is straight forward to setup.
Once you have your mail downloaded to your machine via fetchmail, you could use procmail to do any processing/forwarding etc that you might need, considering you have multiple accounts (OSX, Evolution indicates this).
Your outgoing mail is still handled by your mail client since you are sticking with SMTP.
You can find out more by
www.google.com/linux for
postfix
procmail
fetchmail