Fix "Mail delivery failed: returning message to sender"

Recently, Gmail blocked the emails sent from my domain name. Look like gmail treats our domain name as spam. I need someone who can fix this with gmail.

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 failed:

xxxxxxx @xxxxxx.com:
SMTP error from remote server for TEXT command, host: gmail-smtp-in.l.google.com (142.251.4.26) reason: 550-5.7.26 This message does not have authentication information or fails to
550-5.7.26 pass authentication checks. To best protect our users from spam , the
550-5.7.26 message has been blocked. Please visit
550-5.7.26 https://support.google.com/mail/answer/81126#authentication fo r more
550 5.7.26 information. p138-20020a0de690000000b002dc4be27cf6si8913776ywe.
321 - gsmtp