Bulk mail and Letters

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.

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

Letters noun - Scholarly attainment.

Bulk mail and letters are semantically related. You can use "Bulk mail" instead a noun "Letters".