Do you know that if you are sending an e-mail regularly to your Sphere of Influence, but you are not doing it responsibly, you could be doing a great deal of damage to your domain name's e-mail reputation?
If you're not following up with your SOI consistently, you're losing money in lost referrals and repeat business. If you are, and you're using e-mail as one method of doing so, are you doing it responsibly? If not, you could end up on various e-mail server's blacklists, and that is not somewhere you want to be. That could stop all of your e-mails from getting to that client, and possibly anyone else who happens to be using that server as well. And servers share blacklists sometimes, so it could get worse.
By responsible I mean are you using an e-mail service that provides an unsubscribe link? As long as you are doing that at a minimum you're protecting yourself to a degree. What you really want to be doing though is making sure that if someone unsubscribes or labels your e-mail as spam, that you remove them from your CRMs e-mail list . If you're monitoring your outgoing e-mail with a good service, you'll also know when e-mails "bounce". If a former client's e-mail bounces or is undeliverable, it gives you a good excuse to call them to "update your records".
If you're not already monitoring the delivery of your e-mail you should start. If you have e-mail addresses going back for years that you have never checked, you could have a high percentage of bad ones. If you decide to start doing regular e-mailings and you use those bad ones over and over, that's when the potential for being blacklisted comes in.
Your biggest asset is your past client list, so shouldn't you do your best to maintain it's integrity?