I am using satchmo store and when some one signs up right now it sends a welcome email. However I have no idea what file this welcome email comes from so i can edit the message.
Does anyone know?
You can find it in satchmo_store/contact/templates/registration folder. There should be a file welcome.txt.
All the registration related templates are present in that folder. Although I will highly recommend you NOT to edit the templates in the package itself, but rather override them with our own. Refer to this for details.