My SMTP server (not really under my control), can't send to sourceforge mailing lists:
... while talking to mail.sourceforge.net.: >>> DATA <<< 550-Postmaster verification failed while checking <firstname.lastname@example.org> <<< 550-Called: 126.96.36.199 <<< 550-Sent: RCPT TO:<email@example.com> <<< 550-Response: 550 <firstname.lastname@example.org>: 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 <email@example.com>... User unknown <<< 503 valid RCPT command must precede DATAThis 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.