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I have a question which is not about the configuration but about the question what is right and what not.

The scenario: Organisation A (OrgA) has a Mailserver which is responsible for example.org. The Head of OrgA is also Head of Company B (CompB, example.com). Now the it guys from OrgA used their Postfix aliases file to forward all mails for Head.Boss@example.org to Head.Boss@example.com

Should look like this:

cat /etc/postfix/aliases
Head.Boss:Head.Boss@example.com

Now a worker of CompanyC writes an email to Head.Boss@example.org which gets "redirected" to Head.Boss@example.com. In that case the Mailserver of OrgA acts as simple mail relay, right? If CompanyC provides a SPF record and CompanyB checks this one there should be a failure because the mailserver of OrgA does not match with the SPF of the sender CompanyC.

Is it a good manner to redirect mails to external addresses like that? Or shouldn't that matter because there is no DMARC Record which tells to drop messages not matching the SPF?

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The it people at orga.com should use SRS to forward the mail. For instructions on how to set this up, see: https://jichu4n.com/posts/setting-up-dkim-and-srs-in-postfix/ . For more information, see: https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme

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Company B (example.com) should be aware that the Organization A (example.org) is forwarding the mail. This way the mailserver of example.com could be trusted to perform the SPF/DMARC checks on behalf of the mailserver of example.org and, thus, the SPF/DMARC checks can be omitted.

While the implementation is a technical solution, this really comes to the trust relationship between the organizations in question, as well as whether potentially confidential information about Organization A should be forwarded to and held on the servers of Company B.

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  • But you cannot have this trust relation with every company that is possible to write emails. – Nico Sep 10 '19 at 12:54
  • In the described scenario, you don't have to. Company C has nothing to do with this. – Esa Jokinen Sep 10 '19 at 13:06
  • Ah, you mean a trust between OrgA und Comany B? Well that would help but if thats not the way you should forward mails you should not accept this. – Nico Sep 10 '19 at 13:33
  • Yes. If there's no such relationship between the organizations, just the same individual, the Head.Boss should have two mailboxes. – Esa Jokinen Sep 10 '19 at 13:38
  • Okey, got you... – Nico Sep 10 '19 at 14:35

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