I tend to use a slicehost slice to do my own outbound mail (I use Google Apps for all inbound).

Assuming you have a slice first you'll need to set up your DNS entry pointing to the slice and a reverse DNS entry in the slicehost manager. See these resources for more information:

http://articles.slicehost.com/2008/9/2/mail-server-slice-set... http://articles.slicehost.com/2007/10/24/creating-a-reverse-....

Next set up an SPF record for your domain to legitimize your new mail server:

http://articles.slicehost.com/2008/8/8/email-setting-a-sende...

Finally you'll want to install the mail server software and configure it. I have a blog post that shows the standard config needed to configure the mail server and set up DKIM and DomainKeys to help your mail look legitimate:

http://blog.nimbu.net/setting-up-a-postifx-outbound-mail-ser...

Check out the links at the bottom of the article for more information about DKIM and DomainKeys.