You can basically use any program as long as it sends the proper headers ,and your not trying to "bulk email" people about viagra and other things that are almost always spam -- in which case no matter what you do it will probably not reach their inbox. You can even write your own simple PHP script using mail() as long as you do things properly. Don't give email providers reasons to not deliver your mail .
The emails will all be genuine offering a service, definatley not spam. I just want to send the email to many address's, the easiest way possible, without them being bounced back.
I'd be careful sending them from your own server. If not done properly your IP could get blacklisted.