For the information to be fairly processed one of the following conditions must apply:

there has been explicit consent from the individual to the processing (this is the hard opt-in procedure)
it is required by law to process the information for employment purposes or some other legal requirement
processing is required to protect the vital interests of the individual
processing is required to carry out public functions
processing is required in order to pursue the legitimate interests of the data controller or third parties although this does not apply if it could unjustifiably prejudice the interests of the individual
Special provisions apply to sensitive data, including racial or ethnic origin, political opinions, religious or other beliefs, trades union membership, physical or mental health conditions, sex life, criminal proceedings or convictions. The likely penalty, by way of fine as well as damage to reputation, is such that in such matters it is suggested that legal advice is an essential.

At first glance the obligations of The DPA can seem daunting but the provisions are more or less what good practice would dictate for your email marketing. It is difficult to argue against words such as fair, limited, accurate, secure and necessary.

The Data Protection Act can be found in full at: http://www.legislation.gov.uk/ukpga/1998/29/contents