e-mail relaying without forwarding the entire message?

[Bill Moran]

Steve Camp <steve at camp.com> wrote:

> 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"?

According to RFC-821, it looks like "551 User not local; please try
<forward-path>" should accomplish what you want.

I've never seen this implemented, however, so I don't know if sending MTAs
react to it correctly.

-- 
Bill Moran
Potential Technologies
http://www.potentialtech.com