In the UK there are basic guidelines to suggest whether the email marketing campaigns you are running are legal or not.
The guidelines differ for B2B and B2C lists:
B2B Lists
No opt-in is required therefore unsolicited emails can be sent as long as an opt-out or unsubscribe option is included.
B2C Lists
Recipients must have opted-in and a valid opt-out must be included. The UK Information Commissioner also included an option for older lists that were collected under previous legislation. These lists can still be used where there was an existing relationship between company & consumer (I.E. The consumer has made a previous purchase from the company).
The best place to get sound marketing law advice is at www.marketinglaw.co.uk where solicitors Osborne Clarke provide regular updates on digital marketing law.