What replaced Royal Mail steamers for secret dispatches?

What modern system replaced Royal Mail steamers for carrying protected government communications?

Encrypted communications handle much of the traffic, while diplomatic couriers still move sealed physical material under sovereign protection and established international convention.

Electronic communications now carry much of the traffic once entrusted to mail steamers, but physical material still travels under diplomatic protection. The dedicated couriers described here as "silver greyhounds" carry sealed cases between embassies and foreign ministries without access to their contents. Their protection comes primarily from diplomatic convention and the consequences of interference rather than from a warship escort. Airports move them rapidly and discreetly because a delay or search can create a state-to-state incident. If a courier or pouch is suspected of abuse, the host state can protest and withdraw diplomatic status rather than casually open protected material. The sovereign protection at the heart of the Trent Affair therefore survives even though aircraft and couriers replaced the nineteenth-century maritime network.