Updated Behavior for Dedicated Domain Default Header Fallback

Previously, the Dedicated Header (From Name and From Email) was always used as the primary identity when sending emails through a dedicated domain.

1. Fallback Header Activation Only on DMARC Failure

- The dedicated header will now only apply when DMARC alignment fails (based on SPF/DKIM logic).

2. Fallback Triggered When No Custom Header is Provided

- If users do not specify a custom "From Name" or "From Email", and DMARC fails, the system will automatically apply the fallback header.

To identify emails from a dedicated domain in the past, Dedicated Headers (From Name and From Email) were always used to identify emails from that domain.