What's the best method to perform e-mail marketing?

I am currently interning for a small start-up company involved in real estate. My boss wants to find out about e-mail marketing. He wants to provide clients and brokers with a weekly newsletter. What is the least expensive and most efficient way to do this? I have checked out software such as Send Blaster, and Pimex MailExpress. Optimally, these e-mails would be sent one-by-one to the recipients. I am looking for a method that would not limit my emails per day. Is there such a method? He would like to use his GMail account.

PS – Mac or PC both acceptable.

Thanks in advance!