Subscribe to Maillist
Maillist subscription can be easily added to a wordpress website by using plugin for Mailchimp to interact with your mail account and can then use the emails you harvest for an email marketing campaign.
Rules which should always be adhered to is to allow clients and potential clients to opt out for you mail shot. Once a client opts out honour their decision. From personal experience there is nothing more annoying then getting spam like email from a source you had opted out of.
It is always good to ensure that both the html and text version of your email is easily legible on mail clients.
Mailchimp is a great product which is free to use below a certain amount of subscribers, and the paid version is quite reasonable if you are successful in garnering a large amount of subscribers.