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A message that you sent could not be delivered to one or more of its
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  4bub123@gmail.com
    host alt3.gmail-smtp-in.l.google.com [74.125.200.26]
    SMTP error from remote mail server after end of data:
    550-5.7.26 Your email has been blocked because the sender is unauthenticated.
    550-5.7.26 Gmail requires all senders to authenticate with either SPF or DKIM.
    550-5.7.26
    550-5.7.26  Authentication results:
    550-5.7.26  DKIM = did not pass
    550-5.7.26  SPF [sourceware.org] with ip: [161.97.169.252] = did not pass
    550-5.7.26
    550-5.7.26  For instructions on setting up authentication, go to
    550 5.7.26  https://support.google.com/mail/answer/81126#authentication d9443c01a7336-211057f55d9si2334085ad.254 - gsmtp