Error on changing workflow state

Hi all, I've published a new OP site and it's all working great. The only strange thing is an error when publishing content (or change the workflow state of any article from published to something else). Here's the error:

Unable to find "action" of name "mimemail_rules_action_mail_to_user" with the label "Send an HTML mail to Author". Perhaps the according module has been deactivated.

I've searched everything on the site, the rules section, the workflow section, everywhere... but I cannot find this rule. Where is it (or where it should be?). Thanks to everyone.
Bye.