When reviewing email messages that were analyzed for automatic promotion by Dynamics 365, you see an email message that failed to be created in Dynamics 365 with an InternalEmailReject sync error.
When Dynamics 365 evaluates an email in your mailbox, multiple conditions are evaluated to determine if the message should automatically be created as an email activity in Dynamics 365. If the evaluation results in an InternalEmailReject error, this indicates that the email was determined to be internal communication from one user to another user in Dynamics 365. There is a System Setting ("Track emails sent between Dynamics 365 users as two activities") which determines if both the sent and received email should be tracked resulting in two email activities for the same email message. If this setting is disabled and the email is between users, the received email will not be tracked.
If you want to track both the sent and received email, enable the System Setting for "Track emails sent between Dynamics 365 users as two activities":
Navigate to Settings, click Email Configuration, and then click Email Configuration Settings.
The Email tab should be selected by default. Locate the section Set tracking options for emails between Microsoft Dynamics 365 users.
Check the option Track emails sent between Dynamics 365 users as two activities and click OK.