Email alerts not being delivered outside the domain

Hello all,

I have a web app for external clients (outside our domain). They have to register for an account and when they do an automatic email is sent. When they are approved another email is sent.

Now when they set up an alert on a list or library they don't get an email.

Why do they get the first emails but not emails from the alerts?

I appreciate any help.

I am using SharePoint 2010 Foundation.

Cheers,

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