The particular use case I'm interested in is AWS SES logging, but there are likely other reasons admins may want to add a custom header. For AWS SES logging, in order to collect more detailed email logs, emails must be associated with a ConfigSet by setting a custom header.
Add a custom header to emails as they're submitted to the SMTP server.
If there were a config that could be sent to mod-configuration that would allow admins to set a custom header for all outgoing emails that would be helpful.
Links to additional info