The spool’s own internal backpressure deadline (see
RocksSpoolParams::store_deadline) elapsed before the write was
accepted. Indicates that the spool itself is the slow component,
independently of any caller-supplied deadline; distinguished from
SpoolCallerDeadlineExceeded so the SMTP server can produce a
peer response that points the operator at spool health rather
than at their own timeout configuration.
The deadline supplied by the caller (e.g. an SMTP
data_processing_timeout) elapsed before the spool accepted the
write. Distinguished from SpoolBackpressureTimeout so that
the SMTP server can surface a peer response that accurately
blames the caller-side deadline.
The spool’s load-shedding gate is currently latched. Callers can
recognize this via anyhow::Error::root_cause (e.g. in the SMTP
server) to produce a peer-safe, actionable response instead of a
generic internal-error message. The Display impl intentionally
matches the bland summary returned by Spool::unhealthy_reason()
so that the wire-facing text is identical regardless of which
layer observes the condition.