At 18:59 -0700 on 04/30/2007, Ned Freed wrote about Re: "for" clause
on Received: header field:
> The solution to the matching issue is simple. If you are the ultimate
receiver domain (ie: Example.com for mail addressed to
*(_at_)example(_dot_)com), just clone the message so there is a separate copy
for each Rcpt-To address and place the appropriate address into the
for clause of each cloned message - IOW: Act as a SMTP Server that is
designed to not group multiple Rcpt-To commands into an outgoing
email message even though the Mail-To's point at the same MX Server.
This simple solution only works if you're willing to eat the disk space and/or
bandwidth it consumes. When dealing with large local distributions which for
some companies can run into tens of thousands of recipients for a single
message, this is an absolute and complete nonstarter. Of course you can put a
threshold on the maximum recipients before separate copies are disabled, but
the cases where for clauses are most useful are those involving large, complex
lists.
The above quote was mine. I fail to see where there is any massive
disk space or bandwidth usage if implemented correctly. There HAS to
be some ultimate SMTP Server that is delivering the message to the
user's POP/IMAP Mailbox. How hard is it for THAT SMTP Server (if not
the ones that is handing the message off it it) to put the for clause
into the Received Header as it places the cloned copy of the message
into the user's mailbox? The delivery SMTP Server MUST clone the
message at that point anyway (and it takes no extra disk space or
bandwidth for the INCOMING message). How far back along the chain of
SMTP Servers doing hand-off/relay between the MX Server that accepts
the message and the SMTP Delivery server that places it into the
mailbox (or redirects the message due to a .forward) you place the
SMTP Server that does the actual cloning is an implementation
decision.