Postmaster verification failure - why?

[Anthony Campbell <ac@acampbell.org.uk>]

I'm trying to post to a mailing list I'm subscribed to. The posts bounce
with this message:


Final-Recipient: rfc822; spamprobe-users@lists.sourceforge.net
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Postfix; host mail.sourceforge.net[66.35.250.206] said:
    550-Postmaster verification failed while checking <ac@acampbell.org.uk>
    550-(result of earlier verification reused). 550-Several RFCs state that
    you are required to have a postmaster 550-mailbox for each mail domain.
    This host does not accept mail 550-from domains whose servers reject the
    postmaster address. 550 Sender verify failed


I have put an entry in /etc/aliases and run newaliases:

	 postmaster: ac@acampbell.org.uk

This doesn't help, however. What is wrong here?

Anthony

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