Reports whether the mail transport is REACHABLE right now, distinct from whether the mailer was constructed. Optional: a mailer whose transport exposes no side-effect-free probe omits it, and so does an implementation that does not answer the question at all.

A false means the transport was contacted and did not answer. An undefined means the question could not be asked — never read it as healthy. This is the tri-state every reachability seam in the framework uses (IRedisClient.ping, StorageProvider.isHealthy, IMessageBroker.isHealthy), and it exists so a holder of an IMailer — notification-plugin's email channel, an application's own health indicator — can report the transport without importing mail-plugin, which AI_GUIDELINES §2.2 forbids.