[kome]

I was wondering the same. And other mail clients as well.

[HelenePhisher]

Just tried it, it seems to only work one-way and tags behave like regular folders. So Mailmate shows your web-created tags as folders, but if you apply an existing tag in Mailmate nothing happens in the web version.

Edit: found that discussion on JMAP support in Mailmate: https://lists.freron.com/mailmate/2019-January/010799.html