[feat] Customizable Mailed-By

[junxit]

Describe the feature

When incoming email is forwarded by ForwardEmail, the “Mailed-By” shown by clients (including GMail) shows “mailed-by: forwardemail.net” instead of the original value. This modification to the email obfuscates the origin/provenance of the email and requires the recipient to look into headers for the actual origin.

SendGrid, Brevo (formerly SendInBlue), Amazon SES are examples of providers that do not do this for outgoing emails (especially not transactional emails).

I understand this is normally used to make sure ForwardEmail gets bounced messages (Return-Path field and such), but maybe make it optional/configurable.

Checklist

• I have searched through GitHub issues for similar issues.
• I have completely read through the README and documentation.