You can’t select text from an HTML message in PowerMail in the same way as a text-only message. To select text in an HTML message, double-click at the beginning of your selection, and shift-click at the end to create a hilighted selection.

Alternatively, you may can click on the globe button, which appears at the bottom of HTML messages, to display them in your web browser, where you will be able to select and copy text. You can also deactivate HTML display from the preference, to display the text only version of the message.