I am looking for a user friendly way to have a user to tell the 'server' to either store or remove an email address.

a. My first thought was to use imapsieve for this and have an imap sieve plugin store/remove the email address of the message that is being dragged into a predefined monitored folder.

This would work with any imap client

b. I was thinking about the message Flagged[1]. I am curious how would I be able to detect this (in sieve)? This does not seem to be stored in the source of the message. Although every client seems to support it, it would probably conflict with every day use cases.

c. Then you have these colours and/or categories. But these do not seem to propagate across clients. Where are these even stored?

Any ideas on how this can be done?

[1] https://www.rfc-editor.org/rfc/rfc3501#section-2.3.2