IMAP has a concept of flags/keywords (https://datatracker.ietf.org/doc/html/rfc9051 2.3.2) and there's a whole bunch of servers and clients that support using it in a similar way as labels - Thunderbird, Zimbra, dovecot, etc.
It's not a feature originally built for labelling like Gmail, so it has a few complexities ("Servers MAY permit the client to define new keywords") and enough clients (especially mobile) that don't support it - but they could just like Thunderbird or some webmail clients.
I've used it basically exactly like Gmail labels for years on TB on desktop.
It's not a feature originally built for labelling like Gmail, so it has a few complexities ("Servers MAY permit the client to define new keywords") and enough clients (especially mobile) that don't support it - but they could just like Thunderbird or some webmail clients.
I've used it basically exactly like Gmail labels for years on TB on desktop.