Can someone please help clarify / verify on the following?

MTA Settings
Under Configure -> MTA -> Network, I'm seeing a few settings here but am still not very clear what to do with them.
Web mail MTA Host name
Is this for delivering local mail only (ie not for external mails)?

Relay MTA for external delivery
My understanding after reading the doc is that if my ISP doesn't force me to relay outgoing mails through them, and I have enabled DNS lookup, I can leave this blank?

Inbound SMTP host name
Sorry I know this is explained as "If your MX records point to a spam-relay or any other external non-Zimbra server, enter the name of that server in the Inbound SMTP host name field." but I'm not following. Can someone provide an example?

MTA Trusted Networks
The admin doc says "To set up MTA trusted networks on a per server basis, make sure that MTA trusted networks have been set up as global settings and then go the Configure > Servers > MTA page and in the MTA Trusted Networks field enter the trusted network addresses for the server." However I see out of the box it has default networks setup for the server whereas on a global level it's blank. Does this mean there is a bug with the install software and I have to copy the setting from the server to the global setting?