diff --git a/lib/Email/Outlook/Message.pm b/lib/Email/Outlook/Message.pm
index 8d89946..9e7f327 100644
--- a/lib/Email/Outlook/Message.pm
+++ b/lib/Email/Outlook/Message.pm
@@ -79,6 +79,8 @@ our $skipheaders = {
   "X-MS-Has-Attach"
 };
 
+# See https://learn.microsoft.com/en-us/office/client-developer/outlook/mapi/mapping-mapi-names-to-canonical-property-names
+# for a list of properties.
 our $MAP_SUBITEM_FILE = {
   '1000' => "BODY_PLAIN",      # Body
   '1009' => "BODY_RTF",        # Compressed-RTF version of body
diff --git a/lib/Email/Outlook/Message/Base.pm b/lib/Email/Outlook/Message/Base.pm
index 7797c60..cf978f2 100644
--- a/lib/Email/Outlook/Message/Base.pm
+++ b/lib/Email/Outlook/Message/Base.pm
@@ -69,6 +69,7 @@ my $ENCODING_DATE = '0040';
 
 #
 # Descriptions partially based on mapitags.h
+# See also https://learn.microsoft.com/en-us/office/client-developer/outlook/mapi/mapping-mapi-names-to-canonical-property-names
 #
 our $skipproperties = {
   # Envelope properties