Currently it is possible to have e-mails as inputs or outputs of flows.
Some headers can be set directly, like
CC, but not all: specifically,
References should be updated for email threading to work as expected.
This would allow for flows where an email by Node-RED can be clearly marked as a reply to the email that initially triggered the flow.
An easy way of implementing this is to add those fields to the existing
nodemailer interface, as it implements them already.