Feedback-ID is an additional header required by Gmail Feedback Loop to see aggregated data in Google Postmaster tools. Gmail requires it to include in DKIM-Signature. The following guide explains how it can be configured in OpenDKIM :
Edit opendkim.conf
vim /etc/opendkim.conf
Add the following line :
SignHeaders Feedback-ID
Reload openkdim :
systemctl reload opendkim.service
Reload postfix :
systemctl reload postfix.service
Send a test email with swaks to gmail :
swaks --from postmaster@postfix.io --to xxxx@gmail.com --h-Feedback-ID 123:asd:123 --server 127.0.0.1:25
My test mail results :
Authentication-Results: mx.google.com;
dkim=pass header.i=@postfix.io header.s=key1 header.b=oY1NVInb;
spf=pass (google.com: domain of postmaster@postfix.io designates 45.55.57.182 as permitted sender) smtp.mailfrom=postmaster@postfix.io;
dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=postfix.io
Date: Sun, 12 Nov 2017 11:04:42 +0000
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=postfix.io; s=key1; t=1510484682;
bh=ecGWgWCJeWxJFeM0urOVWP+KOlqqvsQYKOpYUP8nk7I=; h=From:Feedback-ID:From;
b=oY1NVInbieJq4f6IWDjHVT25S5zvazNDtWuso2TYAEEa80kuh/KpR9Z2T+NxRuiV8
FODtj02rXnAUD1eaGFlzaUq7+HP/2teVi2lZo4DJYxr5eePOsY2GOIFpy35VtNAmBv
BL+lxdRiNDqbSwipwZDFMTja+SKk5+0eUOyqaDcg=
To: xxxx@gmail.com
From: postmaster@postfix.io
Subject: test Sun, 12 Nov 2017 11:04:42 +0000
Message-Id: <20171112110442.017500@console.postfix.io>
X-Mailer: swaks v20170101.0 jetmore.org/john/code/swaks/
Feedback-ID: 123:asd:123
This is a test mailing