Just to be clear: Not all email marketing is illegal - and not all is spam.
Spam is where you send unsolicited emails. That's typically what happens when you obtain an email list from a third party. In a few countries, this is actually against the law. In many other countries, it's not illegal, but it's still highly disreputable. Regardless of the law, it's also a poor way of marketing, mainly because it's untargeted.
Sending emails to existing customers is another matter. Not only is this perfectly legal , but it is also an acceptable form of marketing - provided you do it in the right way. That means only sending emails that are relevant to the needs of the customer, not flooding the customer with a vast number of messages, and - above all - only mailing customers who have explicitly agreed to receive them.
So, to return to the orginal post: Yes, it can be an effective method of marketing. But, yes, there are also legal implications to worry about.