Emails Rejected

I keep getting these emails rejections, but not on all emails. Please help...

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

    host gmail-smtp-in.l.google.com [142.251.111.26]
    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 with ip:
    [66.96.189.9] = did
    550-5.7.26 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 af79cd13be357-7b9ac53e1d5si4787409985a.612 - gsmtp