There are certain data protection rules you must follow when carrying out email marketing. See these commonly asked questions:
Can we advertise the products and services of third parties by electronic mail?
If you are offering a 'host mailing' service, you are not disclosing your mailing list to a third party but you are willing, for a fee, to promote their goods and services alongside yours. It is unlikely you could send such messages on a 'soft opt-in' basis because they are not your own 'similar products and services'. However, you could send such material if the individual has agreed to receive it, provided you identify that you and not the third party are the sender.
Can we use third-party email marketing lists?
The law does not stop you using rented email marketing lists. However, you are responsible for any emails you send so make sure that the individuals you are sending the email to gave their consent for their details to be passed to third parties. You should check with the list rental business that they have the consent of the individuals concerned.