The automatic email will not work in wordpress.com
(1) This is how you can offer email updates of your posts to your readers http://faq.wordpress.com/2006/10/11/how-can-i-offer-email-updates/
(2)You can also put your email address in a text widget and display it in your sidebar http://faq.wordpress.com/2007/01/04/put-your-email-in-a-widget/
(3) If you decide to post an email address in a text widget in your sidebar I would offer the following advice to avoid being spammed:
(a) Do not use your primary email address, instead get a throw away free one from yahoo, hotmail, or wherever.
(b) When you post your email address use an email image generator to discourage spam bots from reading it. Here are links to some free email address image generators:
Here is the link to an email address generator that doesn't have to be Yahoo, Gmail, Hotmail, etc. thus it can be useful for those with their own websites.
generator - http://pribadi.or.id/email/
blog article - http://pribadi.or.id/diary/2005/05/12/newmod-email-image-generator/