Fix outgoing email issue

I had someone from here help with our emails going to spam folders... Now, there is the below issue

Can you fix it?

host aspmx.l.google.com[142.250.113.27] said:
550-5.7.26 Unauthenticated email from xxxxxx.com is not accepted due
to 550-5.7.26 domain's DMARC policy. Please contact the administrator of
550-5.7.26 xxxxxxx.com domain if this was a legitimate mail. Please
visit 550-5.7.26 https://support.google.com/mail/answer/2451690 to learn
about the 550 5.7.26 DMARC initiative. f6si3998603otu.73 - gsmtp (in reply
to end of DATA command)