Symptoms
Unable to send emails to Gmail - it is rejected with:
This mail has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM.
Authentication results:
DKIM = did not pass
SPF [example.com] with ip: [203.0.113.2] = did not pass
Cause
SPF and DKIM are not properly configured.
Resolution
Configure SPF and DKIM according to this KB article.
Note: If there are several SPF records, leave only one.