The system whereby messages are transmitted via the post office(noun)
WordNet. Princeton University. 2010.
Use "mail service" in a sentence
"Telegraph companies, express companies, railway companies, the excellent mail service and the like out-despatch any conceivable agent ” except Petherick."
""I'd made such a point of that with Mrs Spears that I couldn't hardly leave before that was finished, but then I got away and I went into Cairns one Wednesday with John Duffy on the Milk Run" - I found out later that he meant the weekly Dakota air mail service - "and so down to Brisbane."