RESOLUTION
A remote SMTP server can be configured in /var/mailman/Mailman/Defaults.py using the following directives:
# SMTP host and port, when DELIVERY_MODULE is 'SMTPDirect'. Make sure the
# host exists and is resolvable (i.e., if it's the default of "localhost" be
# sure there's a localhost entry in your /etc/hosts file!)
SMTPHOST = 'domain.tld'