I’ve recently stumbled across to pretty important limitations of the SendEmailActivity in SharePoint…
1) The From property of the SendEmailActivity doesn’t actually do anything. The message is always sent out from the SharePoint Server’s configured email address. Setting the From property of the activity won’t change where the message appears to be coming from.
2) The SendEmailActivity doesn’t support attachments. As a result you can’t send the item to which the workflow is attached to workflow participants who are not on the system.
If you need either of these functionalities – you’ll have to develop your a SendEmailActivity replacement on your own.