SMTP Call Forward


EQ Forum Admin
Staff member
Hi Bill,

What is your definition of call forwarding?

I have in the back of my head something Microsoft related named similar to call forward but I can't seem to find it with a Google search.

If you are referring to a situation where an incoming mail server that provides a filtering service does a valid recipient check with the destination mail server before accepting and processing the email, then yes, I would say this is a standard process.



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SMTP "call-forward" is a term used to describe a widespread practice
whereby SMTP servers place an incoming SMTP session on hold while they
attempt to use an outbound SMTP session to determine whether or not a
given email address is valid. The theory behind this is as follows:
if an SMTP server responds positively to an SMTP RCPT command
[RFC2821] with a given email address then this potentially means that
the address local-part is valid. One problem with such a scheme is
the lack of efficiency inherent in the need to tear-up and tear-down
an SMTP session over TCP. Also, because these types of SMTP sessions
are not purposed to deliver mail, they typically drop connection after
the RCPT command is processed. This leads to a large number of SMTP sessions which appear in logs to have simply failed for no reason.
This leads to situations in which SMTP transaction logs can no longer
distinguish legitimate network errors from "call-forward" traffic.

SMTP includes a VRFY command which can be used to determine whether
an email address exists. VRFY is not in wide-spread use and suffers
from the same inefficiency concerns described in the discussion on
SMTP "call-forward". Additionally, SMTP agents providing mail
services to a domain are often not authoritative making VRFY requests
potentially unreliable.


EQ Forum Admin
Staff member
Thanks for the link. SMTP Call Forward is a subset of valid recipient checking. It specifically uses an smtp connection to figure out if an incoming email is going to be accepted by the destination mail server. I've never heard of anyone mention the minger protol and UDP checks before on any of the mailing lists I monitor. These days I'd think most people would be using something more efficient than smtp and/or something that already exists within their email environment to make the valid recipient determination such as a mysql db lookup, ldap connection, or even a local flat file database.