[Feature] Email notifications: add custom SMTP headers

[ghost]

I have searched the existing feature requests, both open and closed, to make sure this is not a duplicate request.

• Yes

The feature

When using some third-party transactional mail service they often require to set a custom SMTP header for grouping emails into a collection. It would be nice to add an option set those custom headers when configuring SMTP notifications.

Examples: Postmark, lettermint

Platform

• Server
• Web
• Mobile