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
out right now.
Some additions to the list:
Cisco Multinet SMTP -- server supports pipelining, client doesn't take
advantage of it.
MX -- server supports it, client takes advantage of it.
Process Software SMTP -- doesn't support EHLO and hence no pipelining.
CDC Mail*Hub -- doesn't support EHLO and hence no pipelining.
Thanks are due to Dan Wing for pointing out the first two.