Issues sending email to google

I have recently had numerous and constant issues with your email service not being authenticated and thus anyone with a gmail.com address is not getting my emails! Here is what the response is:

"Remote server returned '554 5.0.0 <[142.251.179.27] #5.0.0 smtp; 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."

Please fix this, it's infuriating to know that I can't talk to my clients because the email server I pay for isn't up to code.

Thank you.