My humble suggestion, ie. what I do:
I have 2 + 1 email accounts (not considering my work account). The two are full fledge email accounts with slightly different targets. The 1 is this one which I use almost solely for mailing list.
I have Dovecot + RoundCube + Apache + SSL on my Centos5 home server (no configured sendmail, so no mail sending) I use this for mail archival purposed. It has dozens of folders going to (for now) a max depth of 2 folders. I move mail there one it has been read and dealt with, sorted into appropriate folders.
So I have as much space as I need + full access to it. My only concern now is setting up encrypted remote backup.