ReferenceError codes
middleware_error
Plugin middleware threw while preparing a send or handling its outcome.
EmailMiddlewareError has code middleware_error, is not retryable, and exposes phase:
before_sendafter_sendon_error
The original exception remains the standard cause.
Hook exceptions do not produce this error because hooks are isolated and swallowed. Only middleware failures are delivery-visible.
A before_send failure happens before adapter dispatch. An after_send failure can occur after the provider accepted the message, so application recovery must not assume the email was unsent.