SMTP Call Forward

Discussion in 'Mail Server Support' started by BuffaloBill, Mar 18, 2010.

  1. BuffaloBill

    BuffaloBill New Email

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    Is "SMTP Call Forward" a good thing? A practice that is frowned upon? In widespread use?

    -Bill
     


  2. popowich

    popowich EQ Forum Admin Staff Member

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    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.

    -Raymond
     

  3. BuffaloBill

    BuffaloBill New Email

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    From http://bgp.potaroo.net/ietf/idref/draft-hathcock-minger/ ...

    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.
     
  4. popowich

    popowich EQ Forum Admin Staff Member

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    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.
     

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