I find myself wondering if *any* other mail clients support the expiry
feature offered by Gnus, whereby you can mark a mail for *later* automatic
deletion (after a per-group-configurable time), and force immediate
deletion of all expired mail if you need to.
This is really, really, killingly useful: it's like having a
multi-day-long perfectly reliable email undeletion facility. I find it
astonishing that nothing else I've tried has done it (not that I've tried
anything else with much hope that it'll be able to exceed what Gnus can