When using Sitecore Email Experience Manager (EXM), a message might be sent with an incorrect sender header value. Sometimes email messages can be bounced with an error similar to the following:
5xy Reject due to policy restrictions (header based)
To resolve the issue, consider one of the following options:
Be aware that hotfixes were built for specific Sitecore XP versions, and must not be installed on other Sitecore XP versions or in combination with other hotfixes. In case any other hotfixes have already been installed on a certain Sitecore XP instance, send a request for a compatibility check to Sitecore Support.
Note that the ZIP file contents need to be extracted to locate installation instructions and related files inside. The hotfixes must be installed on a CM instance and then synced with other instances using standard development practices.