Each of them has a role to play in making the purchase, and each of them needs to be marketed to. Email Marketing campaigns sometime need to be customized for each of the participants of the purchase process. Not every person in the decision making process would be a potential recipient for the same email marketing campaign.
For example, while the CEO of an organization could be one making the final sign-off on a high value sale, it's unlikely that the CEO would like to receive periodical product catalogues from you (indispensable for procurement). However the CEO needs to be aware of your business. Depending on the nature of your business and the size of email list the customer organization, you could definitely get the attention of the CEO with a specially addressed email annually or semi-annually.
Messaging Senior Management: Launch of a new product/service Invites to special events that you could be hosting Seasonal greetings Any Industry news that you possess and could be of business interest to the CEO/Top management Messaging Users, Procurement, and Technical evaluators: You must look at addressing Procurement, Users and technical personnel at frequent time intervals. These people initiate the purchase, are present through all the critical decision points of the purchase process, and quite often have veto power to negative a supplier.