[Dovecot] imap and vpopmail: per-domain auth

Hi, i have qmail + vpopmail + dovecot + squirrelmail 0.99.14 on Fedora Core 4. Qmail includes its own pop3 server, and everybody is able to access it from outside. Dovecot is used only locally by SquirrelMail - so everybody can access via webmail.

I'd like to make imap directly available from outside, but only for a few selected domains (NOT ip addresses!), while leaving everyone able to access with pop3 and/or via webmail.

I don't know if this is possible, and if it is, i don't even know where to start. Is it a vpopmail or dovecot setting?

Thank you for any suggestion and/or pointer to the right documentation.

-- Francesco