Forum Moderators: phranque
I've had Horde and SquirrelMail on a hosting account for a while and I've never been able to find a way to import a PST file or old e-mail into an IMAP system like those without a lot of unseemly work-arounds. Although both of those platforms are certainly functional I find them a bit lacking when dealing with volume mail. Also, if you look at the AJAX alternatives provided by Google, Yahoo and Microsoft you'll see that you can expect a lot more from a web based e-mail account.
If you're comfortable with open source, you might want to take a look at Zimbra as an Exchange alternative. It appears to have a lot of supporters and backing.