Activation Email Only Sends To HostGator Emails

When a user registers on my forum, it is supposed to send an activation email. It used to send one to all users. Now, it will only send one to my domain emails and anyone with a HostGator (my host) email address. So basically, it will only email an activation email to someone within my server network. HOWEVER, if I delete a user, it will send them an account deletion email, no matter what their address is. So, no activation email to a Gmail account, but it will send a deletion email to a Gmail account. HostGator support is receiving an activation email, so they believe it has to do with the plug-in. Can someone please help?

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