1. Look for (your_site_root)/concrete/mail/block_form_submission.php. To override it, copy it into (your_site_root)/mail/block_form_submission.php and edit the new copy.
2. Replace this (line 11):

There has been a submission of the form %s through your concrete5 website.

to

There has been a submission of the form %s through your ". SITE . " website.