The irony abounds. I got a bounce like this on Sunday, when I attempted to cast a "yes" vote for restricting the inkscape-user list to members only.

inkscape-user@lists.sourceforge.net: 66.35.250.206 does not like recipient. Remote host said: 550-Postmaster verification failed while checking <george@...634...> 550-Called: 205.158.62.206 550-Sent: RCPT TO:<postmaster@...634...> 550-Response: 550 <postmaster@...634...>: User unknown 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 Giving up on 66.35.250.206.

If this gets through, my ISP should have added a postmaster account.