My ugly mug

Hi, I'm Thomas Cannon!

I work on Freckle with Amy Hoy and Thomas Fuchs, and I teach people how to add an extra layer of polish to their work. Previously I worked for BMW and Clemson University, and have a bunch of smaller apps I tinker with on the side. Follow me on Twitter!

If you’re building any kind of application or service, you want to make absolutely sure that you don’t accidentally send out emails to real email addresses while you’re developing or testing. There’s no excuse for doing so, and it makes you look bad.

Me holding a sign that reads "Accidentally sent development emails to real customers"
You don’t want to be this guy. Look at him, that’s the face of someone who’s not having a good day.

The problem is: you need to “send” emails and check to make sure they look right, and it would be even better if it “just worked” (while still making sure you don’t make a mistake).

OS X comes installed with Postfix, which is an SMTP mail server. By default, it does have the capability to send emails to external addresses. Using Postfix is great, because it can be used by everything with minimal configuration.

Once we disable Postfix from being able to send emails to external addresses, we can use an application like MockSMTP to preview the email and rest easy that we aren’t spamming people or hurting our reputation.

Checking if it’s possible for email to be sent from our development machine

In order to ensure we are unable to send emails out to external clients, we have to make sure that none of the default SMTP ports are open on our development machine.

Any of the following are considered “default” SMTP ports:

Use the following command to see if any of these ports are open (meaning that email could be sent to the outside world, which is a very bad thing):

sudo lsof -i -P | grep -E ':2525|:2526|:25|:587|:465'

If there are any results, we need to reconfigure our SMTP servers to deny access to the external network.

Disabling Postfix from accessing external networks

Thankfully, it’s very easy to prevent Postfix from accessing external networks. All that is involved is commenting out a few lines in the configuration file.

The configuration file we are looking for is It’s usually stored in the following location:


Open this file and look for any lines that containing inet. For example:

smtp      inet  n       -       n       -       1       postscreen
submission inet n       -       n       -       -       smtpd

Comment these lines out with a #:

#smtp      inet  n       -       n       -       1       postscreen
#submission inet n       -       n       -       -       smtpd

These entries are what allow postfix to access the external network:

inet The service listens on a TCP/IP socket and is accessible via the network. source

Commenting out these lines will only allow UNIX or local-domain ports to be used by Postfix. MockSMTP should still work, and you won’t run the risk of sending out emails to actual customers.

After you are finished, run the following command in the terminal:

sudo postfix reload

Other things to look out for

There are a few other things you’ll want to check before you can rest easy:


After you’ve finished making sure your development machine’s SMTP server can’t send out real emails, you can use the following command to make sure MockSMTP is working (make sure it’s open first!):

date | mail -s testing YOUR_EMAIL_ADDRESS

UPDATE: Configuring your Rails app

Unfortunately, this process breaks devilering emails from a Rails app. My best guess for this is that Rails uses the TC/IP ports, and not the UNIX or local ports. Thankfully, the MockSMTP site include a code snippet you can use.

This goes in config/environments/development.rb:

config.action_mailer.delivery_method = :smtp
ActionMailer::Base.smtp_settings = {
  :address => "localhost",
  :port => 1025,
  :domain => ""