The law in relation to direct marketing is particularly fast-moving. If you want to stay on the right side of it, always tell customers exactly how you want to use their personal information and get evidence that they agree to such use. If you have existing customers on your database and want to send them marketing information relating to products and/or services similar to those you have previously supplied to them, you may continue to do so, but you should always offer them the ability to opt out of receiving further mailings. Note that individuals have a legal right to stop you sending them direct mail marketing at any time.
