After the new attribute MAIL:label was introduced (see also https://discuss.haiku-os.org/t/introducing-a-new-indexed-attribute-meta-tag - where it was first imagined as META:tag), it’s now usable in nightly images with the Mail application and a new Tracker add-on “Label as…”.
Usage is similar to the old “status” concept. If you were using custom statuses before, I suggest to query them and migrate them to the new “label”.
One important note:
When you install a new nightly (or in the future an official release), everything’s set-up, ready to go. If, however, you update and existing Haiku nightly installation, you should create the index for the new attribute before you start using it.
In Terminal, change to the partition you store your emails an enter:
mkindex -t string MAIL:label
Any questions?