James mail queues is handling mail priorities. See the MailPrioritySupport interface.
This is done using the MAIL_PRIORITY mail attribute. It is an integer property ranging from 0 (low) to 9 (high) and mail queue component will default to 5.
Today, the mail processing unit akka mailetContainer can not change the priority set by the SMTP layer (using MailPriorityHandler). This is a problem as we can not re-prioritise outgoing emails, for instance in remote delivery queues.
I would thus propose the following mailet:
Furthermore, we might want to customize processing logic depending on the priority (might it just be for debugging purpose).
To do so, I propose to introduce the following matchers:
How to do this?
In the `server/mailet/mailets` project, in the mailet package create the WithPriority mailet which sets the MAIL_PRIORITY attribute.
Create the matchers in `server/mailet/mailets` in the matcher package.
Write unit tests for these mailets / matchers.
Use MailetUtil content to parse the integer conditions/value. You can reuse https://github.com/linagora/james-project/pull/1215 to ease your work.