best way to reach your past customers and leads (from previous consulting business) about my new software?

Here's my current situation:

I used to own a consulting business. I have many contacts but not sure how I would go about telling them about my new startup beta software.

It's been at least several months since we've done business. When I went into full time development mode, I shut down my consulting business website but google Apps gmail is still enabled. I felt there was no need to continue communicating with them if I didn't have time to provide any useful services.

Here's my dillema:

Should I bother contacting leads whom I fail to close? What about few clients who were not 100% satisfied with my consulting? Should I hit em up again?

Do I send an email from myconsultingbusiness.com or mynewstartup.com ? I'm hesitant to send email from mynewstartup.com because

A) I need to build relationships again, they might know who I am anymore.
B) Not sure if my emails will reach their inbox. Since in the past, Google Apps didn't deliver some of my emails.

In addition, how would I mass send out emails to my contact list in Google Mail? Would Google flag my email as spam when recipient field has couple hundred email addresses?

Should I send email on one on one basis? It will take a lot of time to do this.

Should I use Aweber instead?

Use bcc for sending the email.

Personally, I'd recommend having an open house somewhere and inviting everyone. At the open house you can pitch your software. The open house doesn't have to be at your office, but can be at a restaurant or bar (happy hour style). I've done this with about 10% acceptance (so 10% of the audience became customers).

I personally feel we should always be n touch with old customers and try to update things if you are going to start a new one and if you need them. So my suggestion is that you send an email from the previous email and remind them that you worked with them and tell them about your start up and then go back to them with your present one.