Campaign Monitor

Campaign Monitor is an email marketing software system for designers - and while I'm not really a designer, it has allowed me a whole lot of freedom in what we send out at my company.

I previously used iContact.com after trialling this and ConstantContact.com. I liked iContact customer service, but their packages weren't quite right, and I could never get my campaigns to look very professional (as I was doing them myself).

CampaignMonitor works just great, they even have a $5 per campaign trial option where they will send the DM email out to their own specially set up accounts across different email providers and operating systems and show you in little screen grabs what the display will look like on each system.
This is really useful as what looks good sent to my Outlook 7 may appear very differently in hotmail, or on an iphone/blackberry.

They are also very affordable for sending low quantity campaigns.
Their service is intended to be re-branded for design companies/ PR companies to set up email campaigns for their own clients, and is sometimes white-labelled as something else.

You can send emails out to 5 addresses before you need to pay to, no credit card details needed here, so easy to try it out too.

They have very good customer service for the small issues I've needed help with also.