Hello - I'd suggest using one of the email service providers. You don't have to worry about deduping and keeping your list clean yourself re: opt-outs (the providers' software is generally pretty robust about maintaining this for you); plus they typically have good relationships with the ISPs so email from them is theoretically less likely to get blacklisted.

In my experience, the ISPs also have all sorts of cool widgets & doohickeys you can embed into your site to collect registrations, track behavior and send future targeted mails to, etc. Less stuff for you to build & worry about maintaining.

[edited by: caveman at 12:26 am (utc) on Mar. 8, 2007]
[edit reason] Removed specifics, per TOS [/edit]