Send emails to users I want to send email to the users of my site, individually or to all of them  at once.
Is there any available module?

I am using Drupal 6.
Use this Mass Contact module it will match your requirement.


  The Mass Contact module allows anyone with permission to send a single
  message to multiple users of a site, using its roles functionality.


or

The Advanced User Module allows users who have "administer users" permission to send mail to selected groups of users, and that includes selection by role.