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sender verify callout question

Discussion in 'General Discussion' started by shann, Jun 28, 2007.

  1. shann

    shann Well-Known Member

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    Hi,

    Some of the mails are not reaching to some of the email providers and returning with the
    sender verify callout error.

    What exactly senders callout feature does?
     
  2. sparek-3

    sparek-3 Well-Known Member

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    Sender callouts work by doing a callback to the sending server and trying to verify that an e-mail address is valid before your mail server accepts that message. For example, say you have sender callouts enabled on your server and someone sends you a message. Say that individual is sending you a message from bob@commcast.net. The e-mail transaction, when the message is being sent to your server will look something like:

    S: 220
    C: EHLO <hostname>
    S: 250
    C: mail from: <bob@commcast.net>
    S: 250
    C: rcpt to: <your@address.com>

    (Lines beginning with S: represent your server, the response code that your server is giving to the mail server that is trying to send you a message. Lines beginning with C: represent the client server, what the server that is trying to send you a message is saying).

    At this point your server will perform a callout. It will look up the MX records for the commcast.net server connect to those mail servers and attempt to send a message to bob@commcast.net (don't worry, it never actually sends a message). The commcast.net server will return in the affirmative if it recognizes bob@commcast.net as being a valid e-mail address on its servers. If it does not returns a negative. Depending on what response your mail server gets, at this point of the e-mail transaction your server will either respond with a 250 Code (accepts the message) or a 550 Code (rejects the message).

    If you think about it, it really makes sense. Why should your mail server bother to accept messages from e-mail addresses that don't really exist?

    However, to put things bluntly, there are several, several mail server administrators who are uneducated and either do not have their mail servers configured correctly or think that it is OK to send out e-mail messages from fake addresses.

    For one thing, the callout system users the null sender in its callout attempts.according to RFC 1123, all mail servers must accept messages from the null sender. However, once again, there are several mail servers and mail server administrators who do not configure their mail servers to accept mail from the null sender. If a mail server does not accept mail from the null sender, then callouts from that domain will always fail.

    You also have large corporations who insist on sending out newsletters and other e-mail messages from addresses that do not accept messages. Technically, there is nothing wrong with this. But in my opinion, common sense tells you that if you are going to send out an e-mail messages from bob@commcast.net, then bob@commcast.net better accept incoming mail messages. If commcast.net does not want to read the messages sent to bob@commcast.net, then they should configure bob@commcast.net to immediately delete messages upon arrival. This is just my opinion regarding this issue, but good luck trying to get large corporations to understand this. If someone can explain to me why its reasonable for someone to send out a legitimate e-mail message using an envelope-sender that does not also accept mail, then I will listen. I just don't know of a valid reason at this time, other than technically there is no rule that says you can't.

    If people are writing addresses on your server and receiving messages saying the sender callout failed. Then it either means that their server, the server that is handling mail for the domain from which they are writing the message from, is not handling the null sender properly, they are not sending out e-mail from a valid e-mail address, or there is a connection problem or issue that is preventing your server from contacting their mail server and causing the callout to fail.

    Hope this helps.
     
  3. pross

    pross Well-Known Member

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    Thats a great explanation :) If only more corporations would actually listen to reason and not allow fake addresses, then it would be a more useful option.
     
  4. sparek-3

    sparek-3 Well-Known Member

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    I would really like to see an RFC or some other e-mail standard that would basically say that whatever e-mail address you use as an envelope-sender when sending an e-mail, then that e-mail address must also accept messages. This would give something tangible to point to when an e-mail service or large corporation sends out a newsletter or other message from an address that does not accept messages. There may be a standard or RFC that states this, but I am not aware of it. And its possible that there could be other ramifications of such a standard that I cannot forsee. But right now, when a company such as nytimes.com sends out a news letter from an e-mail address that does not exist, and your e-mail users on your server (that is using sender callouts) do not receive those messages, all you can say is that nytimes.com would appear to be lacking common sense by sending an e-mail from a fake e-mail address.
     
  5. sparek-3

    sparek-3 Well-Known Member

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    Not trying to reopen a thread or bring more attention this thread, but I wrote an article concerning this over the weekend. I thought this might give some useful insight into this problem should someone else run into problems with this.

    http://www.spareknet.org/articles/callouts.php
     
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