Doesn't send email

[khaledkhaled24]

Hello All,
I have a problem with my vbulletin. When I run a diagnostic for sending email inside AdminCP it works fine and send the email in no time. The problem now is, I want to send an email when a user post or makes a new thread. When I enter inside the options of the the sub-forum and write my email in
a) Email Addresses to Notify When there is a New Post
b) Email Addresses to Notify When there is a New Thread

It doesn't send the email. Knowing that when I run the diagnostics it did send a test mail correctly.
So, what is the problem now??

Thanks in advance.

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any help!!!!!!!!!!!!!