You must not send unsolicited marketing e-mails or newsletters. Instead, you generally need to get prior consent from the customer (or potential customer). It is good practice to ask customers and visitors to your website to 'opt-in' to receive such messages.
However, the rules are different if you have an existing relationship with a customer for a similar product. If so, you do not need to ask for consent, but you must at the very least allow them to 'opt-out'. In e-mail newsletters, for example, explain how to unsubscribe from the list.
