Installing SMTP module, not able to enable module
Most likely, your host provider does not have one or more of the PhP extensions enabled for your site.  Check the status page / php info to see if the following items listed in the readme are there:


  REQUIREMENTS
  
  
  Access to an SMTP server
  The following PHP extensions need to be installed: ereg, hash, date &amp; pcre.
  Optional: To connect to an SMTP server using SSL, you need to have the   openssl package installed on your server, and your webserver and
  PHP   installation need to have additional components installed and
  configured.
  


If the requirements are there, check the watchdog error log (you probably have).

If possible, check the actual server error logs.

If you can't check the error logs, you might try going to the Logging and Error settings in the Devel area and setting the option to display all errors with a stacklog.  

NOTE: This should be reset once your debugging is done.