If they are a customer and you are marketing a similar product, while giving them the opportunity to opt out then, in many case, yes!
Where a customer has created an account on your site, direct marketing may be sent to that customer where the conditions below are met:
A person may send or instigate the sending of electronic mail for the purposes of direct marketing where—
- that person has obtained the contact details of the recipient of that electronic mail in the course of the sale or negotiations for the sale of a product or service to that recipient;
- the direct marketing is in respect of that person’s similar products and services only; and
- the recipient has been given a simple means of refusing (free of charge except for the costs of the transmission of the refusal) the use of his contact details for the purposes of such direct marketing, at the time that the details were initially collected, and, where he did not initially refuse the use of the details, at the time of each subsequent communication.