As y@w has already said you can set zimbraMailTransport. Default is for it to be set to
Code:
zimbraMailTransport: lmtp:<zcs_server_fqdn>:7025
And you can change that with
Code:
su - zimbra
zmprov ma user@domain.com zimbraMailTransport:"relay.myisp.com:25"
When you pull down the mail via fetchmail ensure that it is being injected on port 7025 otherwise you will end up with a mail loop.