How to mass email and do not get spam folder - EXPERT questions

I have registered members to my website. I have a really important announcement. I already have C# coded email sending software but emails drops spam folder. What precautions i can take to prevent dropping spam folder. I asked several mass email company about my issue that i want to buy like 350k email service but none of them replied my emails.

The environment is

Windows Server 2008
IIS Server 7.5
Netframework 4.0

I am planning to add only text no link or images. I suppose adding images or links would increase my emails to drop spam box.

You can tell me modifications on my software or on my iis server or i am also open to affordable mass email companies.

1 - you need IP authority to send this amount of emails, look at your on: http://www.reputationauthority.org/ and http://www.senderbase.org/

2 - your mailing must be actual (99% of valid email address);

3 - you need to mail it slowly, look for PowerMTA of port25 solutions.

It is also recommended to use a rotation / variability message body.

As i know,there is some tool can do it.but it cost some money