Email marketing is a highly effective way to develop and maintain a relationship with your client base. As long as you know the client or prospective client and you have permission to email them, you can (and should) do so regularly.
One of the issues lawyers face when using email for legal marketing, is the request of a client or a prospect to unsubscribe from an email list. Many people think this is an indictment of their marketing strategy or of their ability to write compelling copy. That is just not true.
When someone unsubscribes from your email list it does not mean you are a spammer. Many people think that an email that contains a marketing message is spam. It is not. In truth this message may be helpful to many people. If someone opts out, they do so for their own reasons. If you have permission to email them, you do not need to worry about this message being misconstrued as spam.