So, after a week of troubleshooting e-mail bounces, during which time GoDaddy's tech support mainly laid the blame on the fact that my client's IP address comes from a dynamic pool, my client's correspondant dumped GoDaddy and moved their e-mail hosting elsewhere.  This probably was largely influenced by the fact that my client's IP address DOES NOT come from a dynamic pool and is properly registered both in DNS and RDNS.

Tired of banging his head against that wall, the client's correspondant said "Enough of that" and GoDaddy now GoByeBye.

-B-