Some email accounts unable to send email to my qmail server...

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I am running Qmail + Vpopmail on a RH Linux server. I have got authentication and then allow relay to work. If I send email from my Yahoo mail account to one of the virtual domains it works fine every time, also a few other email accounts I have send and receive fine. HOWEVER... if I try and send email from a Hotmail account I have it never ever gets delivered to any of the accounts on my box. I get a report back from the hotmail servers to my hotmail account saying that sending was delayed and then after 5 days it says it's failed. Also, this happens on some other non-hotmail accounts. Why does it not work on certain ISP's and mail accounts?

Thanks in advance,

Simon