Joomla 1.5 has an inbuilt joomla 1.5 mailing component which lets you send out mass emails to your users in html or text format.
The first step to set this up is to select the mass mail component in the joomla administrator.
This will then bring you to the joomla mass mail screen. You will have some joomla options on the left hand side which will look like.
These options allow the following
Send in HTML allows you to send an email in HTML format to allow images and formatting to suit your style
The group option allows you to send to certain user groups only Recipients BCC sends the mass email to the administrator of the website when it is sent out to the users as well