another fix for #1399

(cherry picked from commit 75ba773adb)

src/server/mail/mailRethinkDb.js

@@ -71,7 +71,7 @@ module.exports = {
 						}
 					});
 
-					io.deleteAsync({
+					await io.deleteAsync({
 						key: doc.id,
 						table: 'mail'
 					});