What's an email
For marketing privacy regulations, the definition of 'e-mail' is a message that's sent over a public network which includes voicemail, mobile phone text messages, sound files and image files, plus voicemail and answerphone messages.
When you don’t need permission to send a marketing email
You don’t need to get permission to market to customers you've been in touch with before as long as:
You’re promoting a similar or related product or service
You've obtained the individual’s details in the course of a sale or the negotiations for a sale of a product or service
You must ALWAYS offer them the chance to opt out ON EVERY EMAIL YOU SEND, make it easy for them to opt out or unsubscribe, and make it free to opt out (eg don't use a premium rate phone number)
When you do need permission to send an email
If you contact people who've never bought from you or made an enquiry from you before it's called an unsolicited approach. This applies to both individual consumers, and to named individuals within an organisation (eg Joe@companyx.co.uk). Before sending an unsolicited e-mail you must get permission first otherwise you'll be sending spam emails.
To get permission why not make a simple phone call to ask? Or get contact details by offering a free download from your website?
If you rent or buy data for e-mail marketing, it's your responsibility to check that the list is of people who've opted-in to receive promotional e-mails from companies providing goods or services similar to yours. If they haven't don't waste your time or money buying the data.
For more information see the Information Commissioner's Good Practice Note on electronic mail marketing
What information has to be included in your emails
You must make it clear who the email is from but there's certain contact and company information that by law you also need to include.
See what information to include on your stationery and website