dovecot imap server with indexing
I've run my own imaps server for a while (wu-imap), but it was getting slow mainly due to large files (using mbox mail format). I considered switching to maildir format and I considered a 3rd party host. While researching mail hosts, I got off track (as I often do) and ran across dovecot. The killer feature for me is indexing. My main email client (Thunderbird) is very responsive now. The first access to a mailbox is normal speed, but after it's indexed, access times are seemingly instant. You can read details about dovecot's indexing strategy here.