Postfix(ed)
After much frustration I gave up on using Virtual Mailbox Domains and gave Virtual Alias Domains a go instead, and it worked!
I wanted to set my mail server up like that of a commercial webhost and not have to rely on having to create a local account for each mailbox, but my Linux-fu isn’t that good :(
But I can now send & receive mail from steve {at} <$domain>, job’s a good ‘un!
Now I wish that I didn’t know about the existence of using TLS for encrypted connections to the mail server and using ClamAV and SpamAssassin for server-side content filtering…