First-class and First-class mail

First-class adjective - Of the very best kind.

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

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

First-class and first-class mail are semantically related. Sometimes you can replace phrase "First-class" with "First-class mail", this phrases are similar.