If your ISP provides an e-mail account, just set up a mail program like Microsoft Outlook or Outlook express with the relevant POP3/SMTP or IMAP addresses. You can the set up your groups within the mail program and do bulk postings.
If your ISP doesn't provide an e-mail account you are going to struggle to get the free webmail accounts like Yahoo & Google to do what you want.
You could pay for something like Yahoo's Mail Plus and then use Outlook rather than webmail. The annual subscription is not huge. Or, you could use a free SMTP server for sending your group mails out and just use webmail to check for incomming mail: