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It -is- protocol bloat, and with ESMTP pipelining the extra commands
aren't terribly expensive, so it sounds marginal.
Sendmail 8.9.0 won't have pipelining, so it DOES mean an extra
whole round trip per additional RCPT TO:.
Out of curiosity, what MTA's currently support SMTP pipelining,
and what percentage market share do they currently have in total?
Ones I know of that do include: Innosoft's PMDF, software.com's Post.Office,
ISOCOR's NPlex, SMail, and Qmail. Sendmail, Microsoft's Exchange SMTP enabler,
Lotus Notes SMTP gateway, ISODE's PP, and MMDF do not as far as I know. (I
believe the latter two actually don't implement ESMTP of any sort.) I've heard
mixed reports about Pegasus Mail, possibly as a result of there being several
different SMTP gateways for it. I've heard reports that the USDA's SMTP
implementation in MUMPS supports pipelining, but cannot find an server to check
out right now.
I also suspect that Exchange, NOTES, and PP are "pipeline clean" but don't say
so. Sendmail is the only MTA I know of that isn't "pipeline clean"; I
understand this is an artefact of the way its implemented and not easily fixed.
I have no data on market share, or at least none that I can post here. I can,
however, say that we've observed that pipelining has improved throughput in
BTW, I have no strong feeling either way about this extension. If someone
specifies it I'll implement it in PMDF, if not I won't.