There are a heap of great email marketing service providers out there.

There are three main reasons to use an ESP:

1. Ease of setup - worrying about your own mail server and keeping tabs on it's configuration is something you don't want to worry about: ESPs let you get started without worrying about any of the underlying infrastructure.
2. Better deliverability - most ESPs have fairly strict rules when it comes to uploading lists or adding subscribers. This is for good reasons: using a quality ESP can help guide you through the most basic deliverability issues. On top of this, they generally send millions of emails every day, meaning a few thousand more of yours are unlikely to make a dent. This is good, as your campaigns are less-likely to get flagged as SPAM.
3. Everything in one place - ESPs give you the ability to manage your subscribers, your templates and your results all in one place. This makes creating new campaigns and reviewing old ones much easier.

If you're wondering which ESP to use, I'd recommend trying a few different providers (most have free trials).

If you're solely after the ability to send one-off newsletters and want the sexiest, simplest provider, I'd recommend Campaign Monitor:

If you're after a tool that will give you automated and newsletter campaigns, I'd recommend Vero ( Disclaimer: I'm a Vero co-founder) :