add default mail port to the example file

.example.env

@@ -65,7 +65,7 @@ CUSTOM_DOMAIN_USE_HTTPS=false
 # More info on the configuration on http://nodemailer.com/.
 MAIL_ENABLED=false
 MAIL_HOST=
-MAIL_PORT=
+MAIL_PORT=587
 MAIL_SECURE=true
 MAIL_USER=
 MAIL_FROM=