I'm trying to find out how Zimbra is protected against beeing an open relay server. Normally this is prevented by limit the hosts that are allowed relaying, normally only the hosts on the local network. Right?

Is this the same way that Zimbra is protecting the server? And if zo, is it still possible to send mail trough a normal (SSL) SMTP connection from somewhere on the internet? Or do you have to use the web client then or the Outlook connector?

Could someone point me to a link where stuff like this is described? Or explain it of course.