I noticed exactly the same problem you describe on installing v5b4, but this does not seem to be an sme issue. Rather I believe the MX records may not be configured correctly by your dyn/dns/go resolver (as hinted at by Julie).
My workaround/solution was kinda basic, but it works for me...
I set my domain to the the same as my isp e-mail address e.g. for Phil I would use optusnet.com.au as the domain (reason - at least you can be sure their mx records are correct). All my dyndns domains are virtual, set up on I-Bays, and guess what...
I receive mail no problems, users created on the sme box (and recognised by my ISP as e-mail users) automagically receive mail and holds them until my winblows clients lift them from the qmail server. One tip I will give is that if you wish to use IMP to retrieve mail via the web, set the mail accounts in your winblows clients to IMAP or the IMP inbox will always be empty if a pop client gets there first

Hope this is not off-topic and it helps a little.
Best Regards,
RypPn
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