A large number of SMTP implementations support SMTP pipelining, including: 

(1) Innosoft's PMDF and Sun's SIMS. 
(2) ISODE/MessagingDirect's PP. 
(3) ISOCOR's nPlex. 
(4) software.com's post.office. 
(5) Zmailer. 
(6) Smail. 
(7) The SMTP server in Windows 2000. 

SMTP pipelining has been widely deployed in these and other implementations for some 
time, and there have been no reported interoperability problems.