Some companies use email marketing often as part of their ongoing marketing strategy and for man it works well. There are many ways in which you can target your customers with email marketing. Some companies make each email very specific to the recipient, adding their name and possibly products that they know they will be interested in, whilst other simply send a generic email to all of their database.
If you don’t have a database or have a very limited one you may be tempted to buy in data from a third party. You may get approached by a third party offering you thousands of email address for a few hundred pounds, but be very cautious about doing this. From my experience this does not work and you can actually end up upsetting people (that have not agreed to be contacted by you) rather than making any sort of sales.