TwitterMail is an email integration of Twitter, so you can get email updates from your friends, and post directly to your Twitter from your email account.
You'll get a "secret" email address for sending messages to Twitter, and you'll need to leave the subject line blank for regular posting.. There are a few other features to TwitterMail besides the ability to post from your email account as well. Updates that show on your Twitter page will be emailed to you, and you can also get an email with the last twenty posts from your friends in one message, by sending an email to Twitter with the subject line "Friends." TwitterMail will automatically convert your long URLs to TinyURLs, and messages that are more than 140 characters long are automatically shortened, and include an external "read more" link to view the entire message. You can also set a scheduled time for posts to be sent to Twitter, and any messages sent during Twitter downtime will be automatically published once Twitter is back online.