What a great concept to do for a newborn. Make sure to use a bigger email service like gmail etc. Send them tid0bits, photos, letters from you or family, stories of things that happen to them – give out the email address to Grandmother/Father, Aunts/Uncles, and others that play an integral part of their lives. On their eighteenth birthday they will have such a special time going back and reminiscing about all the special people and events they have gone had in their lives already!
One a side note: Some email services require you to log on every so often to make sure the account is still active. Make sure to log on to the account at least once a month to avoid a problem.