(This applies to sendmail, NOT the zimbra postfix instance)
To set a smarthost, look in /etc/sendmail.cf (or /etc/mail/sendmail.cf) for a line that starts with "DS" and add your smarthost name:
DSmail.bellsouth.net
When you invoke sendmail (use the sendmail installed one, usually /usr/sbin/sendmail.sendmail) add the -Ac option:
/usr/sbin/sendmail.sendmail -Ac
recipient@domain.com
This will force it to use the config file, even for an initial submission.