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value of deferred RCPT responses?

2004-04-03 13:09:03

I find myself wondering how valuable deferred RCPT responses really are.

Here's why I say that: because of a race condition documented in RFC 1047, SMTP servers should try to minimize delay between the receipt of CRLF.CRLF to end the message and the sending of a reply. This further implies that servers shouldn't be waiting to actually deliver the message to recipients, running procmail scripts, etc. before they respond. Having the capability to report separate status for each of several recipients doesn't eliminate the race condition.

SMTP's current design handles two kinds of per-recipient errors:
- those are cheap to test for and can be done with the information available at RCPT time - those that are expensive to test for or which cannot really be tested until after you see the message

I suppose there is a third kind - those that can be caught on-the-fly, while the message is being received, but which due to SMTP limitations cannot be reported until after the message is received. Still I have to wonder how many MTAs are going to get restructured so that all of the testing for each of the recipients is done concurrently while the message is being received. To do this and still minimize the potential for races strikes me as a coding nightmare unless you seriously constrain what kinds of criteria can be applied on a per-recipient basis.


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