e-mail relaying without forwarding the entire message?

[Steve Camp]

When one MTA attempts to deliver an e-mail message to another MTA, the
conversation looks something like:

    -> HELO somedomain.com
    <- 250 Hello somedomain.com
    -> MAIL FROM: <sender at somedomain.com>

    <- 250 2.1.0 Sender ok
    -> RCPT TO: <recipient at otherdomain.com>
    <- 250 2.1.5 Recipient ok
    -> DATA
    <- 354 Enter mail
       ...
       <- 250 2.0.0 Message accepted for delivery

My question involves redirecting the sending MTA to a different
receiving MTA.  Rather than the initial receiving MTA having to accept
the entire message (all the DATA) and then forward it on to another
MTA, the initial receiving MTA simply "redirects" the sending MTA to
another MTA -- preferably the MTA that actually handles the e-mail for
"recipient at otherdomain.com".

The conversation that I envision would look something like:

    -> MAIL FROM: <sender at somedomain.com>
    <- 250 2.1.0 Sender ok
    -> RCPT TO: <recipient at otherdomain.com>
    <- XYZ X.W.V Go See someotherserver.otherdomain.com for <recipient at otherdomain.com>

Is such a redirection possible?

Is it hard to configure?

Have I just described "relaying"?

It seems to me that such a "redirect" would be useful (by keeping
bandwidth low on the primary MX for a domain) if an
organization handles such a large volume of e-mail that it has an
"e-mail server farm", or if a geographically disparate organization
has multiple e-mail servers all connected directly to the internet
(well, probably each through a separate firewall).

Regards,

--
Steve Camp
steve at camp.com