Sending messages directly without using your ISP server is evil. I have pasted below an article I wrote for the FAQ:
When sending e-mail you have the choice between using your ISP, your web host or your own dedicated SMTP server (An SMTP server is a mailserver that you use to send email. It stores your messages and ensures that your messages are either delivered or returned to you with a failure message).
Your ISP usually will provide an SMTP mail server but most ISPs will not want you to send bulk email through their mailservers. Many have restrictions on the number of messages you can send per hour.
Your web host should provide you with a dedicated SMTP server. Particularly if you are a business and you have a legitimate need to communicate with your customers, then your web host should provide SMTP server service as well. They may also have sending limitations, but they are likely much less restricted than an ISP mailserver.
Your own dedicated SMTP server is also an option. You will need to make sure that your IP address is not on any listings of dialup or dynamically allocated IP addresses (Residential IP) and make sure the IP address you are using is registered to a domain name preferably with an MX record.
Note that MaxBulk Mailer has no SMTP server built-in because nowadays it no longer works well with most big ISPs. Some of them like AOL will bounce your messages as soon as they detect they have been sent from a residential IP, an IP that belongs to a range of IPs for DSL/Dialup connections, including static IPs, or that IP has no reverse DNS (PTR) or no domain assigned to it. Following our tests, using your own SMTP server or an application with a built-in server will cause between 20 and 30% additional bounces. The question is, can you accept that up to 30% of your customers will never get your messages?