You can create the email like a normal HTML page and paste the code into Thunderbird (a free email client, sister to Firefox). Of course, you can also save the code as a web page for those who can't view the email properly with a link to it in the email.
Beware, though, as most email clients don't render proper html/css very well. The safest bet is to code like it's the '90s, with tables and only a little bit of inline CSS. Many email clients will rip out any other CSS.
There are also issues with sending out a large amount of email. You can save problems by going with a mailout service like CampaignMonitor.