I don't know exactly who my church has outsourced @urc.org.uk e-mail to, but whoever it is clearly hasn't read RFC822 ("SMTP systems are expected to make every reasonable effort to accept mail directed to Postmaster from any other system on the Internet.")...

fail

Rather difficult to complain about the original broken address I found...

Update: They did get back to me via Facebook eventually. Still not had an answer on why postmaster@ doesn't work, though.