My SMTP server (not really under my control), can't send to sourceforge mailing lists:

... while talking to
>>> DATA
<<< 550-Postmaster verification failed while checking <>
<<< 550-Called:
<<< 550-Sent:     RCPT TO:<>
<<< 550-Response: 550 <>: 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 failed550 5.1.1 <>... User unknown
<<< 503 valid RCPT command must precede DATA

This is annoying and obscure to me, and I have 5% battery left. Ooh, I think I just figured it out, and can fix it. Pressing Post anyway.