The server's NOTES.INI, however, listed the secondary NIC first, as follows:
Ports=TCPINT,TCPEXT
Editing the port order so that the primary port for SMTP mail was listed first (Ports=TCPEXT,TCPINT) and restarting the Domino server resolved the issue.
NOTE: The first port listed in a multi-homed environment will be the port the Domino will use for routing SMTP traffic.
See SMTPNotesPort for more details about the explicit redirection of incomming SMTP mails