Sender Domain Evaluation
The system evaluates both:
The SMTP envelope sender (MAIL FROM)
The parsed From header
These two domains must belong to the same configured parent domain.
Behavior
The SMTP envelope sender is rewritten to match the configured sending domain.
The visible From header is preserved as received.
In practice:
The SMTP envelope sender is changed to a managed bounce/return-path domain under the configured domain.
The From header retains the original sender identity.
The system selects the deepest configured domain that matches the message.
Examples
1. Only the parent domain is configured
Customer configured domain: example.com
Original sender: marketing.example.com
Customer > Relay
MAIL FROM: example.com
From: example.com
Relay > Internet
MAIL FROM: example.com
From: example.com
Explanation
Since only example.com is configured, the relay uses the parent domain for the rewritten envelope sender.
2. Parent domain and subdomain are configured
Customer configured domain: marketing.example.com
Original sender: marketing.example.com
Customer > Relay
MAIL FROM: example.com
From: example.com
Relay > Internet
MAIL FROM: marketing.example.com
From: example.com
Explanation
Because marketing.example.com is configured and is the deepest matching configured domain, the system uses it for the rewritten envelope sender.
3. Both parent and subdomain are configured, the envelope uses another subdomain under the same parent domain
Customer configured domain: example.com and marketing.example.com
Original sender:
MAIL FROM: example.com
From: example.com
Customer > Relay
MAIL FROM: example.com
From: example.com
Relay > Internet
MAIL FROM: example.com
From: example.com
DKIM: d=marketing.example.com
Explanation
The SMTP envelope sender and the From header are under the same parent domain, so the system allows the message.
The visible From header remains marketing.example.com.
The SMTP envelope sender is rewritten to the configured sending domain.
DKIM is signed using marketing.example.com.
4. Only the parent domain is configured; the visible sender is a subdomain
Customer configured domain: example.com
Original sender:
MAIL FROM: example.com
From: example.com
Customer > Relay
MAIL FROM: example.com
From: example.com
Relay > Internet
MAIL FROM: example.com
From: example.com
DKIM: d=example.com
SPF: example.com
Explanation
Since only the parent domain is configured, the system rewrites the SMTP envelope sender to example.com.
The visible From header remains marketing.example.com.
DKIM is signed with the configured parent domain, example.com.