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Newsgroups: mod.protocols.tcp-ip
Subject: When to acknowledge SMTP messages
Message-ID: <8702271841.AA15482@hermes.ai.mit.edu>
Date: Fri, 27-Feb-87 13:41:34 EST
Article-I.D.: hermes.8702271841.AA15482
Posted: Fri Feb 27 13:41:34 1987
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   Date: Thu, 26 Feb 87 16:47:02 PST
   From: jordan@ucbarpa.berkeley.edu (Jordan Hayes)
   Organization: Experimental Computer Facility (XCF), UC Berkeley

   Kevin Crowston writes:

	   I decided that the client SMTP would have to wait while the
	   relay delivered the message.  Otherwise, the relay could
	   acknowledge the message and then crash or discover that the
	   destination mail server was unable to take the message.

   Sendmail seems to handle this correctly, since "delivered" to that part
   of the code means "placed in the queue" (i.e., wrote it to disk ... if
   the machine then crashes, the daemon will pick up where it left off
   since the queue file is still there) -- you can't acknowlege the
   message as being sent before you have firm control of it.  That's what
   lock-step is all about.  Once you have done that, if you find later
   that you can't deliver it, it's up to the recipient SMTP process to
   send it back to where it came from.  This can be handled
   asynchronously.

   /jordan

Actually, sendmail doesn't handle this completely correctly. Before
sendmail queue's up a message, and gives the acknowledgment back to
the sender, it attempts to expand every address in a mailing list.
This expansion can take a long time, since it means a call to the
resolver to qualify host names. So, messages sent to large mailing
lists take a long time to get queued up. What sendmail should be doing
is writing out a very simple queue file with the un-expanded
receipients. The background delivery process should do the expansion
the first time it comes across an un-expanded address.