Emails sent through 'mail.protection.outlook.com' are rejected as 'Access Denied'

Symptoms

Email sent from your server does not arrive at its destination when the recipient domain uses Microsoft's mail servers, and an entry similar to the following may be found in the exim_mainlog log file.

SMTP error from remote mail server after RCPT TO:<example_user@domain.tld>: 550 5.4.1 Recipient address rejected: Access denied.

Description

This issue is caused by Microsoft's anti-spam policies, which have not been made public.

Workaround

Enroll the sending domain in Microsoft's "Junk Email Reporting Program." This program provides the sender with a report on emails marked as "junk" or "phishing" so senders may take corrective actions.

Additional resources

Outlook.com Postmaster: Sender Solutions