You have mainly three options to send the emails.
1st - Which of the most of the people think is to send email via Gmail smtp, but it will not work at all. Forget about gmail to send emails. Why? The asnwer is simple. Suppose you send email XYZ to 100 people using one gmail account smtp, then the same email to another 100 people and after you have sent say few hundred emails, gmail will think that it is spam and will not send the email and will bounce back the email to your spam folder of gmail.
Q. What software can you use to send thousands of emails using multiple gmail accounts?
Ans. I guess you are thinking that if you use gmail SMTP then other mail serviced like hotmail will think that it is coming from Gmail and will let the email go into inbox. Well, that isnt the case, when you send emails using SMTp your IP ADDRESS IS EXPOSED VIA HEADERS, so anyone who received it will be able to TRACE BACK TO YOU. So, gmail isnt the good option at all.
2nd - Send via running our own SMTP Servers on your own PC.
Q. How to create a smtp server on vista?
Ans. Get OK Smtp Software, or QK Smtp server.
But when you send emails via SMTP server running on your own PC or send email via Gmail SMTP from your own PC, your IP Address will get exposed in the email header and anyone will be able to trace back to you.
3rd - Sending emails via Hosting
This is inst very easy at all. Because hosting site doesn't allow you to send more than 300 emails per hr normally and normally when you cross 1000s emails, they will delete your hosting account.
So, now you have to decide which option to use, try to use the combination.