4.8 Complaints about EHLO
We've got a postmaster at some site complaining to the effect that, "Your server machine keeps connecting to our GenericWare SMTP server and sending EHLO. Isn't this an error?" (Note: this complaint will generally occur only when you are running the no-longer supported LSMTP mailer with LISTSERV.)
No, it is not. The SMTP protocol (RFC821 et seq) dictates that SMTP servers must answer "500 Unrecognized command" (or similar) when they receive a command they do not understand. The newer ESMTP protocol, introduced in the early 90s, relies on this assumption for proper operation. After receiving a 500 reply to the new EHLO command, the server will send a normal HELO command and the transaction will continue using the original SMTP protocol.
SMTP servers which close the connection when they receive an unknown command are in violation of RFC821 and are unable to communicate with ESMTP servers. Closing the connection looks exactly the same, to the ESMTP server, as a network failure. Since the server thinks the network connection has failed, it enqueues the message for retransmission.
The solution is for the problem site to modify their SMTP server to return a 500 error code and not disconnect when receiving an unknown SMTP command. We understand that in some cases this may not be an option. For instance, they may be using a very old product whose vendor may have gone bankrupt or may have decided not to make the change because they are phasing out the product, or whatever the case might be. However, please understand that the current versions of most SMTP products use ESMTP, and that people want to use ESMTP because of the improvements it offers over the original (1982) SMTP protocol. In other words, this problem is not specific to L-Soft; it is specific to the software the site is using. ESMTP-compatible servers are commonly available, commercially and as freeware, for most operating systems.
If it is absolutely necessary to turn off ESMTP support in LSMTP for a given host, you can create a "mailer" in LSMTP (see Configure/Mailers in the WinLSCP GUI) for the host in question and disable EHLO (and thus ESMTP) in the "Protocol" section for that mailer entry. L-Soft does not recommend turning off ESMTP globally in LSMTP, and would recommend doing so only in cases where the recipient host is adamant that they will not accept your mail with ESMTP.
It is HIGHLY UNLIKELY that this is an issue any longer in the late twenty-teens, but we are leaving this FAQ in place just in case.