Right then, I used the Virtual Mailhosting howto on the Gentoo site. I've now IMAP, POP3 and Webmail in clear and over SSL, all in all very happy.

Now I've get everything working except the virtual users. Every time I try to send a mail to a vuser I get an email from Postfix saying it can't find the user (which is because it's not a real user on the system). Could someone else who uses this to explain what they do when they want to add a virtual user, I'm sure it's something really easy I'm missing.