Answer:

PONYEXPRESS

AnswerCrossword Clue
PONYEXPRESSEarly US carrier
PONYEXPRESSHorseback post
PONYEXPRESSOld mail system
PONYEXPRESSOld Missouri-to-California mail system
PONYEXPRESSDelivery service of old
PONYEXPRESSOld West conveyance
PONYEXPRESS19th-century mail system
PONYEXPRESSShort-lived mail system
PONYEXPRESS1860s mail service
PONYEXPRESSLegendary mail service
PONYEXPRESSForerunner of the Iron Horse.
PONYEXPRESSPostal route of 1860.
PONYEXPRESSAncestor of airmail.
PONYEXPRESSThey were seldom left at post
PONYEXPRESSIt delivered in the Old West
PONYEXPRESSMail service made obsolete by the transcontinental telegraph
PONYEXPRESSEnterprise for which a 14-year-old Buffalo Bill worked
PONYEXPRESSEarly mail system
PONYEXPRESSSaddlebags of mail
PONYEXPRESS19th-century mail delivery service
PONYEXPRESS19th-century mail transport
PONYEXPRESSDelivery service of yore