Hi Mike Wylde,
Regarding the selected messages are they ones that you have written? For example you're able to choose the priority for an email when composing it using the steps below found in the article here: Write and send emails in Mail on Mac
"Write an email
- In the Mail app
on your Mac, click the New Message button
in the Mail toolbar (or use the Touch Bar). - In your message, add people you want to send it to.
- To use other fields, such as Bcc or Priority, click the Header button
, then click a field. - Enter the subject of your message, then add your text."
If you're looking to organize or flag incoming emails, the articles below can help.
Flag emails in Mail on Mac
Sort emails in Mail on Mac
Filter emails in Mail on Mac
Cheers.