Ideal way to exchange mail with external hosts? Then a HOWTO?

[zcook]

Zimbra does a fine job by default of getting mail sent to and from different addresses on the local domain, but what about external domains like yahoo, gmail, etc?

Should the ISP's smtp address be set as the MTA? Using the web inteface? A CLI tool? Hand config editing?

What if the ISP's smtp requires authentication?

How about hooking in pop3 accounts? Then, once the pop3 account is pulled how to make it appear as though the pop3 is what is sending it back out if the user replies?

Let me try and spell out what I mean to avoid any confusion.
User logs into zimbra as user@domain.local and then their pop3 account user@publicdomain.com is pulled in.

How can it be set up that when they send an e-mail to AnotherUser@domain.local their own domain.local address is listed as the reply-to address.

Then when they send out to a public address, their pop3 is listed as the reply-to, and their local address is not.