Don't be afraid to use email as a way of building customer relationships and SELLING your products and services.  Just do it wisely, as already pointed out in this thread.

Not all unsolicited email is illegal, and not all unsolicited email is unwanted (SPAM).

I have consented to receive marketing emails from many companies, but I do not solicit the individual contacts.  This is consensual, unsolicited email.  Something the Information Commissioner's guidance tries to explain (unsucessfuly) with the idea of buying a round of drinks at a bar.

In essence, I have agreed (consented) to receive email marketing from a company but I do NOT request (solicit) specific content or information from them e.g. "Please, Easyjet, would you send me some information on any 1p flights to Scandanavia, as I am interested in taking a holiday there."

These are the companies you trust to send you emails which are well-targeted i.e. stuff you want, and not too frequently.

So, let's get the ducks in a line. You can always push your content (unsolicited email) to corporate subscribers e.g. duncan.smith@compnay.co.uk, until such time as the individual you are marketing to opts-out (section 111 Data Protection Act 1998).  To-date, there is no legally required screening versus an e-mail preference service.  You are NOT likely to sell much, and you ARE likely to get black-listed very quickly if you push too frequently and with poorly targeted content.

You CANNOT always push your content (unsolicited email) to individual subscribers e.g. duncan.smith@hotmail.com without their consent.  The Privacy and Electronic Communications Regulations 2003 (PECR) makes this ILLEGAL.

How can you get their consent? (we can, and do spend hours discussing this on training courses so don't expect a full answer here, but ...) You can ASK for consent. The opt-in tick box in conjunction with a well-formulated privacy statement will do the trick. If you forgot to ask, it is still possible that your existing customers (finally got to the question!) can be marketed to on the basis of a 'soft opt-in'. Which amounts to an honest and well-founded expectation that your existing customers would anticipate receiving email marketing from you because they gave you their email address in the past, as part of doing business with you, and you're only trying to get them to but more of the same sort of 'stuff'. (there are other finer points of law, but this answer is already lengthy!)

Hope this helps. We have produced a flowchart  which helps explain PECR for marketers, accessible here.

Duncan Smith