This article describes the default behavior of the X-Envelope-To header for locally delivered messages in regards to BCC functionality. The BCC function is intended for the BCC recipients not to have visibility over the other recipients.
The X-Envelope-To header is inserted for locally delivered messages only. If an email is addressed to multiple people, both internal and from outside of the domain, the X-Envelope-To header will be inserted into all sent emails. However, only the internal recipients will see all recipients of the email, including the BCC'd ones.
The external recipients of the email will only see their email listed under the X-Envelope-To header.
The X-Deliver-To header field is designed to deliver the email to the first recipient only while the X-Envelope-to will deliver it to all recipients. Please refer to the RFC 5322 Internet Message Format article for further information.
As this might pose specific issues to some companies, the option can be disabled as per the procedure below:
Navigate to Configuration > Advanced options > Miscellaneous tab.
Disable the radio button for Insert X-Envelope-To header to locally delivered messages.
When sending an internal email, the X-Envelope-To header will no longer be presented.