New answers tagged apple-mail
Use RubiCode's RCDefaultApp (http://www.rubicode.com/Software/RCDefaultApp/) to configure the default handlers for different "protocols" like http:, mailto:, etc. Not 100% positive that you can disable the protocol handler altogether, but you can definitely redirect it to something other than Mail.app.
They will release the follow-up feature for Apple Mail next week (Tuesday). https://www.feingeist.io/mailbutler/
Another question over on Stack Overflow should have enough AppleScript to get you started. You could have this script run in the background, parsing out the sent date of the items in the folder you mention, and give you an alert popup or some other action. I'm also aware of a commercial product, MailButler, which explicitly has follow up reminders as you ...
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