Spam is fought and filtered so many different ways. Some based on content, some based on sender headers, mail format, IP source, rDNS. The list goes on and on.
There is a lot available on best practices, most importantly, the validity of your list recipients and their true (opt-in) desire to receive the message. Googling for things like SPF, safe sender, etc. may be helpful.
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Offering High Quality Dedicated Servers.
Is there a site that lets you send emails to your subsribed members without them receiving it as spamm, or a service that allows you to be whitelisted at YAHOO AND HOTMAIL (most important to me) without making you pay thousand of dollars that you cannot afford?