What IMAP config to use with a relayhost?

[yxz]

I've set up a relayhost for Postfix, though it is only for SMTP, what about IMAP? I can send/receive mail with postfixadmin and external mail hosting, but I cannot connect through Roundcube's test login, when I try 127.0.0.1, localhost or my domain (say mail.mydomain.com).