So, let me rephrase this in a coding term :)
You have couple of links which lead somewhere from your HTML template, to profiles like this:
index.php?action=senddirect&user=2 (or ID of your own account)
So when directly emailing those profiles, you want Guests would be able to post, right?
But you want to exclude Guest emailing to other accounts.
If so, you could add a little switch to bb_plugins code of this add-on, replacing
to something like:
if(isset($_GET['user'])) $user=(integer)$_GET['user']+0; elseif(isset($_POST['user'])) $user=(integer)$_POST['user']+0; else $user=0;
if($user==1 or $user==2) $directEmailGuests=TRUE; else $directEmailGuests=FALSE;
Specify user IDs properly in the bolder code.
Let me know if it worked.