I'm trying to configure my server (zimbra.example.com) that will be accessed using the alias "mail.example.com" and am having trouble getting outgoing mail delivered. This has to be easy, but I'm getting confused (maybe it's late).

I need to have all mail sent go through my ISP's mail server (mail.isp.com). I have listed that address in the "Web mail MTA hostname" and "Relay MTA for external delivery:" options. However, sent mail from the web ui ends up in the deferred queue showing an error of "connect to zimbra.example.com: connection refused"

Can anyone shed some light here? I'm also not sure why it is connecting to zimbra.example.com and not mail.example.com. However, it's more important that mail get forwarded to the 'mail.isp.com' address.