Unable to send emails to Gmail: Gmail requires all senders to authenticate with either SPF or DKIM.

Symptoms

Unable to send emails to Gmail - it is rejected with:

PLESK_INFO: 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.