How to send 100,000 emails weekly and avoid being considered spammer

First of all I'd read Jeff's article on configuring your mail infrastructure DNS bits:

So You'd Like to Send Some Email (Through Code)

Next, you may need to consider signing up with a bulk mail service that many ISP's provide. Whether you're sending mail from your broadband connection or your hosting environment, most ISP's will limit the volume of mail you can send on a daily or hourly basis.

One email or a million, it really makes no difference what software you use. Quite simply, any software capable of sending a single message is capable of sending as many as you like. For the mailing lists I look after I use a simple Perl script.

The software you use is not even a factor in whether you're considered a spammer or not. Just some of the things that do matter are:

• A mailer that doesn't try to fake any message header,
• Proper DNS configuration (reverse pointer and SPF record).
• No virus/malware in the message, or anything that might even trigger a false positive. It only takes one to get you on a blocklist.
• Every message goes out to someone who has voluntarily signed up to the list.
• Use DKIP/Domainkey signing.
• Don't send by CCing to long lists of recipients. One to address per message.
• etc., etc.

Fail on any of those, especially the third, fourth and fifth points, you can pretty much expect to be listed as a spammer, with no sympathy from me.

Additionally, would recommend checking your Email system against this Email Server Test

Another useful site to test settings is MXToolbox