Just like physical letters, an email message has two sets of addresses: the envelope address and the message header address. The envelope address, like the address on the outside of an envelope, is used by the MTA to route and deliver the email message; the message header address, which is part of the message header, is similar to the address attached to a salutation at the start of a physical letter.
The Sender Filter Settings screen enables you to choose the type of sender addresses Trend Micro Email Security uses to match the approved or blocked sender list.
The following options are available:
Message header addresses
By default, both options are selected. Trend Micro Email Security uses both addresses for matching. The Message header addresses option can be modified while the Envelope addresses option cannot.
If Message header addresses is selected on the Quarantine > End User Quarantine Settings screen, Trend Micro recommends you also select it on the Sender Filter Settings screen. Otherwise, the approved or blocked senders added by end users will not work as expected.
Trend Micro Email Security provides the capability of inserting an X-Header in the message header for email messages matching approved senders. If you select the Insert an X-Header in the message header if an approved sender matches check box, you can do extra actions based on the message header on your own MTA or mail server.
The following X-Header is inserted in the message header once an approved sender's envelope address matches:
The following X-Header is inserted in the message header if an approved sender's envelope address does not match but the message header address matches:
Unless specified otherwise, Trend Micro Email Security considers the envelope address as the common sender address.
Regardless of your sender address settings, IP reputation-based filtering and unknown sender domain check will always use Envelope addresses rather than Message header addresses to match the approved or blocked sender list. Unknown sender domain check refers to the check that verifies if the sender's envelop address has a valid DNS A or MX record.