Suddenly, users are no longer receiving quarantine digest notifications. Additionally, emails sent internally are failing to be scanned by MailEssentials. This is a recent occurrence and no changes have been made to either the mail server or to MailEssentials. MailEssentials indicates "Transport" as the error for many of these.
The internal emails are incorrectly identified as inbound and blocked by the Anti-Spoofing filter. To fix this issue, please follow the steps below:
- Add the local domain to the whitelist as described in the Solution section of the linked article.
- Ensure that all your SMTP server IPs are added to the Anti-Spoofing filter, as this is required to ensure the local domains are not blocked.