Emails sent to Gmail Addresses bounce with "no such user"

When we send emails to some people who are using GMail/Gsuite email, messages often bounce with a "no such user here" message: 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 email to them, the message gets through. We've double-checked SPF, DKIM & DMARC and they all seem to be ok. ReverseDNS checks (via MXToolbox) all pass. Not sure how to fix this.