1. make sure that Alert Central have no problems with connection to your host e-mail.
you can test connection by Settings >> Getting Started Wizard >> Next, and then try to pass "Email Settings Test".
2. login into your Alert Central e-mail account and check folder "Sent items". There must be all your notification messages. Also please notice if folder "Inbox" is empty. If not -- then Alert Central not processing income messages.
Are you using POP3 email account? Because there is issues when you configure more than one instance (for example Alert Central and Outlook client) to monitor that email. Only one instance actually can get access.
Thanks Stepan. Somehow even though we were receiving e-mails there was a Authentication issue with Exchange. Fixed and Validation e-mails are being sent out.