If I make a mail server can I deliver bulk email?

Hi, I help a small company, and we have fallen under the fad of "e-mail projects" a.k.a. a Junk mail. Up until now, the business has actually gotten a
customer list from our website, and paid a good chunk of modification for a emailer program. The trouble is, Our list has close to 4,000 people on it and expanding. With gmail just permitting 100 emails per account through on SMTP,

and I am on a tight spending plan, so I cannot hire anybody else. I was considering doing a specialized mail server off of the website server we have been running in the office. Is it possible to make e-mails on your own server, then deliver it with your own SMTP? If it is, exactly what software would I need and whether it free or low cost at least. We run a WAMP server. I set it up just for info, but I could switch it to lamp or whatever if need to be. Thank you for your time and your answers.