When you change your categories, Sortmail offers to apply the change backwards. What that costs depends entirely on what you changed.
Changes that re-read nothing
- Renaming a category, changing its colour, or reordering the list. This is metadata; nothing is re-read.
- Adding or editing a rule. Sortmail rewrites labels from the index it already holds.
- Adding a category the stored fingerprints can already separate.
Changes that need more
A genuinely new category — one whose definition the existing signals cannot separate — needs more than a lookup. Re-sorting works from the fingerprints Sortmail already keeps rather than re-reading your mail; only a full backfill, which you ask for explicitly, goes back to your mailbox, and the fair-use policy says how those are paced.
New mail is never queued behind a re-sort. Whatever you are re-sorting, messages arriving right now are still sorted immediately.
Mail older than the first sort
A re-sort only reaches mail Sortmail has already sorted. Mail that was more than 30 days old when you connected was never sorted, and no re-sort or backfill goes back for it. See Your first 24 hours → Older mail for how to clear it yourself.