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Inbound mail to a specific user is been forwarded to his external mailbox at another domain. Add mails are routed correctly with the exception of one domain. When mail are sent to users of my domain, including that user, the mails are delivered to everyone except him...as the mail admin I am getting the message

The following recipient(s) cannot be reached:

  usermailbox on 1/10/2011 1:27 PM
        There was a SMTP communication problem with the

recipient's email server. Please contact your system administrator. email.speedimpex.com #5.5.0 smtp;587 [email protected] sender domain does not match SPF records>

I am sure this has nothing to do with my mail server, however, can someone confirm?

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You are trying to forward emails from a domain that has SPF records, but that domain does not list your server as a valid source of email for that domain.

Your choices are:

  1. Contact somedomain.co.uk and tell them you're forwarding their mail and want to be added to their SPF record.
  2. Contact the "external mailbox at another domain" and tell them you're forwarding email to one of their users and to whitelist you in their SPF checking.
  3. Use Sender Rewriting Scheme on your server to reflect that you are resending this mail from the original sender.
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  • I am running exchange server 2003...I came across this msexchange.org/articles_tutorials/exchange-server-2007/… however I am not sure if this will fix the problem. If I rewrite the mail going out to this user mailbox would he be able to reply to them at their domain?
    – Saif Khan
    Jan 18, 2011 at 2:47
  • @Saif: that looks to be something different. microsoft.com/downloads/en/… says its basically for mergers, it replaces the from addresses entirely with the new company's address, not for hiding the sender's domain in a way it can be used. libsrs2.org/docs/mta-users.html says an Exchange extension is still "in progress" so your user will have to whitelist you in their "external mailbox". Or send mail directly from "somedomain.co.uk" to "external mailbox" rather than through you.
    – DerfK
    Jan 18, 2011 at 13:58
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Your mail server is not a fault here, but there is a conceptual fault in SPF, that breaks email forwarding.

Explanation: In this case your mail server is a standard non sender rewriting forwarding service, that is in the mail delivery path. This is completely valid according to RFC_822 (page 18).

The SPF Council prescribes that the ESP that is receiving the email must consider possible Forwarding Services and not reject emails delivered via forwarding services only because of an SPF fail. It states: "For non-sender-rewriting forwarders, accept all mail without checking SPF (any SPF results are meaningless)". See the official SPF website at: SPF: Best Practices/Forwarding

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