For a while now I've noticed that when I'm at home doing Drupal development, emails from my local development environment don't get sent. Then, I'll get somewhere else and - all of the sudden - all the backed-up emails will get sent out. I figured out that it's because my localhost had to be configrued to send the email via my Internet service provider's SMTP server. In my case, the ISP is Cox. I was able to make a quick configuration change, and now things are working great.
/etc/postfix/main.cf and find the section containing
relayhost configuration. Just add a new line that looks something like this (replace with your ISP's SMTP server info):
relayhost = smtp.cox.net
Done! This was all it took for me. Depending on your ISP, you may also need to configure port or authentication information. In that case, you'll likely want to look for some additional documentation.