V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

ESMTP
       Extended Simple Message Transport Protocol (SMTP, RFC 1425/1869)


The Free On-line Dictionary of Computing (30 December 2018):

ESMTP

    Extended SMTP.  Initially defined in
   RFC 1869 and extended thereafter.

   See also ETRN.

   (1997-11-21)