Hi again!
Glad that information helped!
What do you mean by checking from where the email has been triggered from? Do you mean the location of the user or by what application?
Otherwise, there is not a straightforward case to send custom values (such as custom headers) or custom data. Also, as you have mentioned, by updating an email template via the Management API or through the dashboard under Branding → Email Templates, these will be tenant wide changes, not application wide. A “temporary” change would be to update the required template via the Management API, send the email then revert the changes to the default template, however, this is not a practical approach.
Regarding you last question, please review our documentation on configuring associated domains, let me know if you need further help on this matter!
Kind Regards,
Nik