First class and Messenger

First class noun - Mail that includes letters and postcards and packages sealed against inspection.

Messenger and first class are semantically related. in express mail topic. In some cases you can use "Messenger" instead a noun phrase "First class".

Messenger noun - One that carries a message or does an errand.

First class and messenger are semantically related. Sometimes you can replace phrase "First class" with "Messenger", this terms are similar.