diff --git a/swaks-tests/html.raw b/swaks-tests/mime-html.raw
similarity index 100%
rename from swaks-tests/html.raw
rename to swaks-tests/mime-html.raw
diff --git a/swaks-tests/nonmime-html.raw b/swaks-tests/nonmime-html.raw
new file mode 100644
index 0000000..7f45889
--- /dev/null
+++ b/swaks-tests/nonmime-html.raw
@@ -0,0 +1,8 @@
+Date: %DATE%
+To: %TO_ADDRESS%
+From: %FROM_ADDRESS%
+Subject: Swaks HTML
+MIME-Version: 1.0
+Content-Type: text/html; charset="UTF-8"
+
+This is a test of <b>HTML</b> at the <i>top</i> level.
diff --git a/swaks-tests/run-tests.sh b/swaks-tests/run-tests.sh
index b89ec0e..4a9044b 100755
--- a/swaks-tests/run-tests.sh
+++ b/swaks-tests/run-tests.sh
@@ -7,7 +7,10 @@ export SWAKS_OPT_to="swaks@inbucket.local"
 swaks $* --h-Subject: "Swaks Plain Text" --body text.txt
 
 # HTML test
-swaks $* --h-Subject: "Swaks HTML" --data html.raw
+swaks $* --h-Subject: "Swaks HTML" --data mime-html.raw
+
+# Top level HTML test
+swaks $* --h-Subject: "Swaks Top Level HTML" --data nonmime-html.raw
 
 # Attachment test
 swaks $* --h-Subject: "Swaks Attachment" --attach-type image/png --attach favicon.png --body text.txt
diff --git a/web/mailbox_controller.go b/web/mailbox_controller.go
index a2d2dbf..79a1a89 100644
--- a/web/mailbox_controller.go
+++ b/web/mailbox_controller.go
@@ -210,6 +210,7 @@ func MailboxHtml(w http.ResponseWriter, req *http.Request, ctx *Context) (err er
 		return err
 	}
 
+	w.Header().Set("Content-Type", "text/html")
 	return RenderPartial("mailbox/_html.html", w, map[string]interface{}{
 		"ctx":     ctx,
 		"name":    name,