IMAP access problem through firewall

Hi
I installed a new firewall (a modified version of the DHCP firewall from
Oskar Andreasson's iptables tutorial). I have a few IMAP mailboxes that I
access from a laptop from inside the local network. IMAP mailbox A is
readily available (indicating that name server lookups, IMAP port stuff is
working). However, IMAP mailbox B is consistently not available. If I try
to access either from outside the network(either directly on the machine
that has the network firewall or elsewhere), both are consistently
available.

This is puzzling. Do you have any suggestions about what I should be looking
at ?

Thanks,

MS