Notification Mail for Private Message Box is almost full - Not working

[induslady]

Hello,

Am using vB V4.2.1.

I understand that there is a phrase - "pmboxalmostfull" that is used to send notification mail to users when their PM box is 90% full as below -

Dear $touserinfo[username],

Your private messages box on $vboptions[bbtitle] has reached 90% of your allowed quota. To ensure that you do not reach 100% of your allowed messages, you should consider archiving or deleting some of your messages. Please visit this page to do so:
$vboptions[bburl]/private.php

All the best,
$vboptions[bbtitle]

But this notification mail is not hitting my users, is it a default feature? can you let me know which setting do I turn it on to enable this notification mail.

Please note that the other notification mail that users get when PM box is full ("pmboxfull") is hitting my user's inbox.

Thanks.