There is an app called MailTags for Apple Mail. It allows you to enter a keyword for every mail you receive. That keyword then appears in the sidebar and lets you easily sort your emails by different keywords as well as use those keywords for search.
Would be great if you could integrate this kind of tagging into Airmail
I used MailTags with Apple Mail also - and I really miss that feature. If there was any way AirMail could read the tags from Apple Mail, that would make it even more useful.