domain questions before installing zimbra

[m3tr0g33k]

Zimbra is fine and looks like making a big difference to my admin tasks.
However.
Before it gets given to the users, I need to work out how to forward mail outside through an MTA.

I have a zimbra server called zimbra.xxx.co.uk (xxx to protect the innocent).
I want mail to seem like it's coming from xxx.co.uk, not zimbra.xxx.co.uk.

Do I need an external MTA for this for forwarding all mail?
If not, how do I change the local hostname so that email addresses and so on are sent out as name@xxx.co.uk, rather than name@zimbra.xxx.co.uk

The hostname must be zimbra.xxx.xo.uk so that we can have an externally resolvable CNAME.

Interested to know if I'm close to the solution or far away

Thanks in advance

[mmorse]

Welcome to the forums,

Before it gets given to the users, I need to work out how to forward mail outside through an MTA.

You can set 'relay for external delivery' very easily via the admin console GUI!

I want mail to seem like it's coming from xxx.co.uk, not zimbra.xxx.co.uk.

This is going to depend on whether your external ip resolves to domain.co.uk or if your talking about some advanced cloaking the server name in the message headers.
We're gonna need a better rundown of where your internal dns comes into play. ie draw us a diagram (but if you just plan to use a relay/a mailserver that's already in existance there's no point wasting time.

If your mx record is mail.domain.co.uk, and that's the only name you ever want the outside world to see, just set that as your servername in the first place in your operating system, then use local dns resolving or virtual hosts to access via zimbra.domain.com

so that email addresses and so on are sent out as name@xxx.co.uk, rather than name@zimbra.xxx.co.uk

This is very easy
To keep the addresses from containing the 'zimbra' or 'mail' just make another domain in the admin console gui!

Don't delete the zimbra.domain.co.uk (or mail.domain.co.uk) -you will still get status/daily reports to the admin@server.domain.co.uk (but you can move the actual admin user to the main domain and just make admin@server an alias. You'll see how easy this is, trust me-dont' delete the 'extra' domain until you're 100% positive it wont affect anything. I just leave it there permanently.
If you've already made accounts just change their assignment to that new domain-you can switch users among domains with a simple drop down.

Your going to be more concerned with A & MX records than CNAMES.