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Postfix spoofed messages using an alias which forwards emails back to the sending domain

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manifestor

My Postfix installation has the following alias configured which will distribute incoming emails to the following addressees :

alias@my_domain.com - goes to - me@my_domain.com 
alias@my_domain.com - goes to - co-worker@our_org.com

If I send an email to this alias using a SMTP account associated with my_domain.com everything works as expected, both me@my_domain.com and co-worker@our_org.com receive the email.

But there is a problem when someone sends an email from an account associated with our_org.com (e.g., someone@our_org.com, which is a Microsoft SMTP server): me@my_domain.com will receive the email, but co-worker@our_org.com not. The reason is that the sending email server of my_domain.com will forward the email using a FROM header that says someone@our_org.com, which he is not allowed to send as, and that’s leading to the MS SMTP receiving email server rejecting the message as SPOOFED.

Now all I want is to get this scenario working:

user_a@our_org.com  → alias.my_domain.com → user_b@our_org.com

Postfix should not forward/send emails using FROM headers he’s not allowed to use, thus pretending to be someone he isn’t. Does anyone now how to configure Postfix to forward the email properly?

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