Webmail MTA hostname:

This is the hostname for your server. Since Zimbra can be clustered, and you can have multiple servers each one should (ideally) have unique hostname. eg: mta1.myzimbra.tld mta2.myzimbra.tld
This setting should match your unix hostname (for RHEL located @ /etc/sysconfig/network)

Relay MTA for external delivery:

Usage scenario: Your Zimbra is on your LAN behind a NAT. You use it primarily to send mail in your organization/company, but your network admin allows connections only to ONE specific external SMTP server. Your e-mail is john@mycompany.tld and you want to send an e-mail to john.doe@gmail.com, you need to specify external MTA relay hostname. So it goes: your internal zimbra->external MTA->gmail

Inbound SMTP host name:

For example, you would set this in the event that you have another server doing your spam and virus checks. Your MX points to that other machine that does all that for you and then pushes it to Zimbra. You need to set that hostname so Zimbra knows where your e-mail comes from.

MTA Trusted Networks:

On a Zimbra cluster (multiple servers, one for ldap, other for mta, third for store, fourth for proxy) you should not use global settings for trusted networks. Trusted networks can, depending on your configuration, relay without authentication and do some other stuff that non-trusted IP's cannot. If you're using monolithic Zimbra conf (everything on one server) you can use either global, or specific settings.