As an admin I want to set specific header for email notifications

Q1. What is the problem that you are trying to solve?
As an administrator, I need to be able to set specific email headers for the email notifications sent by Passbolt.

Q2 - Who is impacted?
Potentially we won’t be able to send email notifications without this possibility.

Q3 - Why is it important and/or urgent?
Using certain SMTP servers (for instance with AWS SES) you are required to add specific headers to your emails.

Q4 - What is your proposed solution? (optional)
Currently we tried changing the ‘EmailTransport’ > ‘default’ configuration to include headers but this does not seem to be taken into consideration by Passbolt. Digging into the code it seems the value for email headers is directly set into the code. Is this the case?

Ideally there should be an environment variable allowing the administrators to set additional headers (next to ‘Auto-Submitted’ => ‘auto-generated’)