Air mail and Bulk mail

Air mail noun - Mail that is sent by air transport.

Bulk mail and air mail are semantically related. In some cases you can use "Bulk mail" instead a noun phrase "Air mail".

Bulk mail noun - Mail consisting of large numbers of identical items (circulars or advertisements) sent to individual addresses at less than 1st-class rates and paid for in one lot.

Air mail and bulk mail are semantically related. You can use "Air mail" instead a noun phrase "Bulk mail".