Okay, you need to define a smart host in your Sendmail configuration then. Take a look at /etc/mail/sendmail.cf and you should find something like
Code:
# Relay all non-local mail to the "Smart" relay host (DS) via smtp: #
# #
# This macro is optional. If you wish to send all non-local mail #
# to a smart relay via SMTP just set macro S (DS) to the name of the #
# smart relay. If this macro is not set, then all mail addresses to #
# a non-local user will be send to the relay. #
#
# "Smart" relay host (may be null)
DS
so to relay any external email through your ZCS server just use
Code:
DSfqdn.myzimbra.server
and then restart Sendmail.