I answer this question based on my own experiences plus the methods used by a few other people I know of within the MR / Consulting industries. I hope you find it useful.
I have used bulk mailer software / online services like Campaign Enterprise, Mach5, SendGrid, Zoho, Mail Chimp, etc.
Generally you have to pay for these in some way, shape or form.
Mail Chimp does have a free service available but there are some restrictions that may or may not suit your needs?
Campaign Enterprise is owned by a company called Arial (based in Wyoming in the US). Back in the day, they had a very affordable package for the small guy, but their pricing structure has changed somewhat and has become more expensive over time. A great package but not sure if the costs suit you?
I have had some experiences with the other software mentioned from time to time. These are best researched to see the finer details.