While editing a newsletters You can send newsletter at the last tab Step 5: Send newsletter. The upper part is to send to the selected mailing lists/user groups, the lower part is for sending for testing.
Repeatedly the question appears: I don't like the texts which are provided by BwPostman. Am I able to change them?
Yes! Joomla!® is able to provide such, so BwPostman is able to do so, too.