At the moment we are using an external e-mail provider and Mozilla Thunderbird installed on every computer as mail client.
We want to use Zimbra now, because it is gives us a great webbased mail/groupware client that also could be accessible from the internet by external employees.
How can i configure Zimbra to use the external e-mail server?
Zimbra should receive the mails from the external e-mail server, do spam-filtering and should use the external SMTP to send e-mails. Nothing more and nothing less. Is this possible with Zimbra?

Thanks for help!