Outgoing mail to Gmail bouncing - Please help!

My email to gmail are bouncing back. 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 [142.250.102.27]
    SMTP error from remote mail server after end of data:
    550-5.7.26 This mail is unauthenticated, which poses a security risk to the
    550-5.7.26 sender and Gmail users, and has been blocked. The sender must
    550-5.7.26 authenticate with at least one of SPF or DKIM. For this message,
    550-5.7.26 DKIM checks did not pass and SPF check for [pluperfecttravel.com]
    550-5.7.26 did not pass with ip: [66.96.188.9]. The sender should visit
    550-5.7.26  https://support.google.com/mail/answer/81126#authentication for
    550 5.7.26 instructions on setting up authentication. bp15-20020a170907918f00b0096a8202e209si1169554ejb.955 - gsmtp