This is a Gmail throttle error that occurs when you fire VERY many IMAP commands at once. Senly's bulk-move endpoint has a retry layer: on "Command failed" it tries 3 more times with backoff (300ms → 900ms → 2.7s + jitter).
In most cases those retries resolve it without you noticing. Do you still see a "N failed" message? Click delete again — the second attempt almost always succeeds, because the throttle has by then been lifted.
If it really happens structurally (more than 5x on one day during normal use), send a screenshot to support.