The old RMS St. Helena has been retired due to opening of an airport on the island of St. Helena.
She will now serve as a floating armoury in the Gulf of Oman:
RMS Queen Mary 2 doesn’t count? Actually, even QM2 isn’t the last:
In recent years the shift to air transport for mail has left only three ships with the right to the prefix or its variations: RMS Segwun, which serves as a passenger vessel in Gravenhurst, Ontario, Canada; RMV Scillonian III, which serves the Isles of Scilly; and RMS Queen Mary 2. QM2 was conferred “RMS” by Royal Mail when she entered service in 2004 on the Southampton to New York route as a gesture to Cunard’s history.
I’m guessing he means actual working mail ship.