Using your standard email client (e.g. Outlook, Hotmail, AOL MailSM, Yahoo etc.) to send bulk emails thru a Bcc text or HTML message to your list is not doing email marketing well.
They are not designed for bulk email marketing and therefore has significant limitations that can actually undermine vs. aid your email marketing efforts. Many email clients and most ISPs limit the number of emails that you can send at one time and guard against bulk emailing, as it is often spam.
Approved Bulk Mailer services typically have strong relationships with ISPs and are able to send bulk email. With many services, while your email address is listed as both the "from", and "reply to" address, the email is actually being sent by the email marketing service's server which is recognized by the ISPs, therefore higher delivery rates
Because email marketing service providers are responsible for ensuring their customers' compliance, they worry about the law and ensure that their services are up-to-date on compliance.