Thanks DrByte!
Problem is resolved. Here's the fix . . .

ZC155a Configuration > E-Mail Options settings > SMTP Email Mail Host

Instead of setting the SMTP Email Mail Host to mail.mydomain.com where "mydomain" is the domain containing the Zen Cart store,
I set SMTP Email Mail Host to mailserver.bluehost.com where "mailserver" is a specific mail server name provided by Bluehost.

DrByte, please let me know when you get a BitCoin account so I can send some love.