Hello Maiid97,
Can you send emails to users on the same server?
If you can send emails to the users on the same sever, I suggest you go to check your SMTP connector configuration.
How to configure the SMTP connector in Exchange 200x
http://support.microsoft.com/kb/265293
If all the users (included internal or external users) cannot receive your email, please post the NDR information in your next post.
Thanks,
Evan
Evan Liu
TechNet Community Support
