Emails sent to Gmail Addresses bounce with "no such user", sent to others fine.

(Sorry for the double post - can't figure out how to delete the old one) Done a bunch of googling but so far no joy. When we send emails from cPanel/Exim to people who are using GMail/Gsuite email, messages often bounce back with a "no such user here" message, even though the email address is correct. The bounce message is: ---- gmailuser@TheirDomainNameThatUsesGmailGsuite.com No Such User Here Reporting-MTA: dns; stan.ourdomainname.com Action: failed Final-Recipient: rfc822;gmailuser@TheirDomainNameThatUsesGmailGsuite.com Status: 5.0.0 ---- If I use another host (like mac.com) to send emails to the email address (gmailuser@TheirDomainNameThatUsesGmailGsuite.com), the message gets through. We've double-checked SPF, DKIM & DMARC and they all seem to be correct via MxToolbox and a couple of other test sites. ReverseDNS checks (via MXToolbox) all pass. We've tested email via all the spam checkers as well - they all pass. Not sure how to fix this.