What is meant by "hard mail adress"?

Yes “hard mail“ refers to physical mail that is postal mail.

Same as “hard copy” of a document, that is a printed version of it, rather than a version that is stored on a computer.

From: Interview Follow Up Guide for the Perplexed: The Career Artisan Series.

Communicating with Potential Employers—Hard Mail or Email—Which is Best? It would perhaps seem counterintuitive to send hard mail in this age of technology. However, the studies myself and my colleagues have conducted in the recent years shows that hard mail is hands down more effective than email when it comes ...