diff --git a/asciidoc/asciidoc-pmg.conf b/asciidoc/asciidoc-pmg.conf
index 45ea7dd..6c9f0bb 100644
--- a/asciidoc/asciidoc-pmg.conf
+++ b/asciidoc/asciidoc-pmg.conf
@@ -21,7 +21,10 @@ clamav=https://www.clamav.net[ClamAV(R)]
 debian=https://www.debian.org[Debian]
 debian_nonfree_archive_area=https://www.debian.org/doc/debian-policy/ch-archive#the-non-free-archive-area[Debian's non-free archive area]
 tts=http://www.template-toolkit.org[Template Toolkit]
+smtp_rfc=https://tools.ietf.org/html/rfc5321[RFC 5321]
+rfc_5322=https://tools.ietf.org/html/rfc5322[RFC 5322]
 dkim_rfc=https://tools.ietf.org/html/rfc6376[RFC 6376]
+dmarc_rfc=https://tools.ietf.org/html/rfc7489[RFC 7489]
 ifndef::docinfo1[]
 author=Proxmox Server Solutions GmbH
 email=support@proxmox.com
diff --git a/pmgconfig.adoc b/pmgconfig.adoc
index 38c7a3a..8035923 100644
--- a/pmgconfig.adoc
+++ b/pmgconfig.adoc
@@ -585,6 +585,18 @@ Controls whether all outbound mail should get signed or only mails from domains
 listed in `/etc/pmg/dkim/domains`, if it exists and `/etc/pmg/domains`
 otherwise.
 
+Select Signing Domain::
+
+Determines whether to DKIM sign emails using the domain found in the envelope
+from or the from header in the body of the email. The envelope from is also
+known as reverse-path, see {smtp_rfc} section 3.3 and {rfc_5322} section 3.6.2.
++
+The envelop from of certain emails, bounces for example, can be empty. In these
+cases it is desirable to sign them using the domain found in the from header.
++
+Additionally, DMARC (see {dmarc_rfc} section 3.1.1) needs the domain found in
+the from header in certain situations.
+
 These settings are saved to the 'admin' subsection in `/etc/pmg/pmg.conf`,
 using the following configuration keys: