Sending an email blast only goes to certain users A little background on the problem:  I have been tasked with figuring out a problem in a Drupal site that I have no experience with.  I have done Web Development using PHP, etc. but never used Drupal.  I have a parent site with 3 sites underneath, and roles for each user (can be members of all sites or just one).  I have a rule that sends an email out to these users after certain content is submitted.  However, not all of the users receive the emails (about 150-180 users per email blast).  

Does anyone have suggestions as to how I can go about debugging and figuring out this problem?  I have to fix this by tomorrow.  Here is what I have done so far:  

-Made sure users were assigned to the role in the email rule.
-Unchecked the box "send only to active users"

Any help is appreciated!
Default PHP mailer has a lot of issues, and sometimes mails are not received or accepted by the servers as this a common method for sending spam.

A good practice is to use the SMTP module (https://drupal.org/project/smtp) for sending emails. You need to connect it to a server (eg. google) for authentification and then it replaces the default drupal php mailer.