Windows Server 2008 R2 Email internal domains

I'm currently using a windows 2008 server with IIS, running PHP as a fastCGI module (don't ask me why lol). Anyhow, the issue is, when using the mail function through php, I can send emails to absolutely any email address EXCEPT to a domain which is also configured on the same server. eg I can send auto emails to example@hotmail.com fine, but not to site@mydomain.com. I'm using parallels plesk with 1and1 hosters.

Does anyone know what I am supposed to do to get around this? I have heard the word relay thrown about loosely, but with my limited knowledge of server administra开发者_C百科tion, I'd rather be sure of what to do

thanks

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You probably need an MX record in your internal DNS so that IIS SMTP knows where to send them.