Hi Nancy,
Great question!
If someone sends you an email about their business that you didnt request, that is spam. Emailing them back with information
they didnt request is spam too. You want to be careful with anything "spammy" because this can get you in big trouble.
To build a mailing list, you need to have a form on your website that invites your visitors to subscribe. You can look at the main page of this site as an example:
http://www.selfstartersweeklytips.com
I use a service to manage my mailing lists: Aweber. You can get a free trial if you would like to check it out:
http://sswt.aweber.com . This is not something you want to manage manually for two reasons. 1) because it will be A LOT of work, and 2) because you have to follow strict guidelines so as not to get in trouble for 'spamming'.
Those guidelines include using a "double opt in" type of list - meaning that your subscriber has to verify that they subscribed to your list or asked to receive your emails (after they sign up). You also have to include an "opt out" link at the bottom of each email so they can unsubscribe anytime. Among other things.
Is the site in your Signature the one you want to build a list for?