[Dovecot] Bit of a Newbie Question

[Jud]

Apologies in advance - I'm sure there's something quite obvious I'm not
doing correctly, but since I'm unable to resolve the problem myself....

I've installed Dovecot 99.14 on FreeBSD-CURRENT using the Dovecot
FreeBSD port.  My "network" consists of a cable modem connected to a
router which is in turn connected (only) to my computer.  My ISP assigns
a dynamic IP address.  I have ddclient set up to sync the dynamic IP to
my hostname and MX via dyndns.org.

Local operations (local delivery via Postfix or copying/moving/deleting
with Thunderbird) work fine.  Mail delivery from remote machines does
not seem to work via either IMAP or IMAP-SSL.  (That is, test e-mails I
send from this [Fastmail] account to my local IMAP server never reach
their destination.)  Enabling port forwarding on my router for ports 993
and 143 has made no difference.

What should I be doing to make this work correctly?

Jud