diff --git a/mail/components/extensions/test/xpcshell/messages/alternative.eml b/mail/components/extensions/test/xpcshell/messages/alternative.eml
index dda8ee906b..11de6a87d6 100644
--- a/mail/components/extensions/test/xpcshell/messages/alternative.eml
+++ b/mail/components/extensions/test/xpcshell/messages/alternative.eml
@@ -7,6 +7,9 @@ Mime-Version: 1.0
 Content-Type: multipart/alternative;
 	boundary="=====================_714967308==_.ALT"
 
+This message is in MIME format.  The first part should be readable text,
+while the remaining parts are likely unreadable without MIME-aware tools.
+
 --=====================_714967308==_.ALT
 Content-Transfer-Encoding: quoted-printable
 
diff --git a/mail/components/extensions/test/xpcshell/messages/sample07.eml b/mail/components/extensions/test/xpcshell/messages/sample07.eml
index 453c289a7c..29283b2ce0 100644
--- a/mail/components/extensions/test/xpcshell/messages/sample07.eml
+++ b/mail/components/extensions/test/xpcshell/messages/sample07.eml
@@ -7,6 +7,9 @@ Mime-Version: 1.0
 Content-Type: multipart/alternative;
 	boundary="=====================_714967308==_.ALT"
 
+This message is in MIME format.  The first part should be readable text,
+while the remaining parts are likely unreadable without MIME-aware tools.
+
 --=====================_714967308==_.ALT
 Content-Transfer-Encoding: quoted-printable