In CRM 4, to send a mail to a custom entity we used to create a new email attribute for that entity and then had to write a custom workflow activity or plugin for sending mail or some complex logic.
http://social.microsoft.com/Forums/en/crmdevelopment/thread/634aa49d-d8fc-4a8c-b3af-6c00b72f8df9
Good news is that in CRM 2011 we have a new feature to Send e-mail for custom entities.
Just need to enable that option in Entity Customization form. It will create a new email field for that entity or will use an existing email field if it is already there.
Bye.