IMAP keywords are basically labels. IIRC they even have Thunderbird integration. You can "tag" messages in TB with a variety of hard-coded tags (like "Work" or "Todo") and they will be saved as IMAP keywords on the server.
The problem is that it's hard to create custom ones in Thunderbird. It involves editing about:config, and when custom ones are saved to the server, other instances of TB won't recognize them unless you edit the about:config of those instances too. Basically, it's possible but too much pain in the TB UI to be useful.
The problem is that it's hard to create custom ones in Thunderbird. It involves editing about:config, and when custom ones are saved to the server, other instances of TB won't recognize them unless you edit the about:config of those instances too. Basically, it's possible but too much pain in the TB UI to be useful.