This article addresses the issue of internal emails getting flagged as inbound and blocked by the Header Checking filter.
MailEssentials recognizes internal emails as inbound when it is not installed on the Exchange server or when the same Email Domain is spread through mail servers physically situated in different locations such as in hybrid setups having Exchange on-premise and Exchange Online (O365) using the same domain.
The Header Checking Anti-Spam filter can be configured to make several different checks on an email, mostly related to the email’s headers. It can also be configured to detect particular groups of inconsistencies in MIME/SMTP header values, as described in the Configuring Header Checking article.
The first step is identifying why the Header Checking filter blocked the message. Once that is complete, the required configuration changes must be applied to the filter, as outlined in the Configuring Header Checking article, or the sender itself must be whitelisted, as these issues can only be resolved by the sender.