My drupal site isn't sending email for some reason. What are some ways to debug this issue of mail sending?
Use a module that allows you to route email through a real SMTP server.
What is probably happening is that Drupal is sending the mail, but it's being blocked as a spammy looking email message.
edit: you can Alphabet for "drupal smtp module", but this is one I've used https://www.drupal.org/project/smtp
The Mail Logger module will tell you if the mail is being sent by Drupal and in fact is a useful module for keeping track of outgoing e-mail.
If you are not getting any email from any contact forms on the site or even registration emails. You might want to goto the server terminal and try to see if it has anything to do with your mail server. You can run the following command:
drush php-eval "print mail('email@example.com','Test subject from drush','Test message','From: firstname.lastname@example.org');"
Just replace the emails with your own. If you still aren't receiving anything you can check the maillogs located here: /var/log/maillog
Then search for the email address you are sending to. If its working it should look something like this:
Aug 11 10:31:56 YRELX05 postfix/smtp: 6222BB1: to=, relay=172.22..[172.22..]:25, delay=0.02, delays=0.02/0/0/0 , dsn=2.6.0, status=sent (250 2.6.0 <20150811143156.627C922BB1@YRNYEELX05.localdomain> Queued mail for delivery)