WordNet sense 1 (any particular collection of letters or packages that is delivered):

post

WordNet sense 2 (the system whereby messages are transmitted via the post office):

mail service, postal service

WordNet sense 3 ((Middle Ages) flexible armor made of interlinked metal rings):

chain mail, chain armor, chain armour, ring armor, ring armour, ring mail

WordNet sense 4 (send via the postal service):

get off

WordNet sense 5 (cause to be directed or transmitted to another place):

send