if you want "Email notifications" to be active by default, when user sends a post (the user won't have to chek the box every times he sends a post) :

on "bb_fonctions.php" file, on "emailCheckBox()" fonction, on line 283 (for minibb1.7d) :
simply replace :
$checkEmail = "<input type=checkbox name=CheckSendMail> <a href=\"{$indexphp}action=manual#emailNotifications\">$l_emailNotify</a>";

by :
$checkEmail = "<input type=checkbox name=CheckSendMail checked> <a href=\"{$indexphp}action=manual#emailNotifications\">$l_emailNotify</a>";

You may want this by some special and individual reason, but to have it on by default is very boring sometimes. I felt it with phpBB in a forum long ago. I had forgotten this option turned on and I had 100-200 unusual letters to delete every day till I remembered to switch it off :)

Yes Ivan, You 're right !
The main reason is not about the Admin but about members : they usually don't check if email notification is active or not when they post.
On standart minibb version, users will probably wait for an email notification they will never receive but there will exist an answer to there post ! That why I think it's interesting activing email notification by default.

Anyway, when I send a post as Admin, I manually desactivate the email notification every times.

I continue to think that it's not needed excepting in some special cases. Imagine - here I could answer you only "Yes", and for this "Yes" you'll receive a special letter with this very important information :)

If I have something really important to tell you in my answer, I can then check the box and you'll receive a notification. For me it's enough.

Anyway, you have found a solution for yourself :)

For Mini BB v 2.3