We are a private detective agency and one of the services that we offer is mobile fingerprinting. Our technicians go to the businesses and fingerprint the employees. Most of our clients are insurance companies, financial services, non-profit organizations and governmental agencies, to name a few.
Are you asking which email marketing provider to use? or how to build a campaign? or where to find your target market?
For email marketing providers, there are many options, but for a free service, I'd recommend MailChimp. They are pretty easy to use, but you have to create/send each email campaign manually.
For automated campaigns (drip or sequence campaigns), I use LeadOutcome. They have a very robust service for the price. You can check them out with my affiliate link nickypink.leadoutcome.com
If you are interested in a drip campaign for your services, you can write the emails yourself or hire them out. I know that LeadOutcome has that service, as I'm sure others do or you could hire a copyrighter that specializes in email campaigns.
The hardest part is creating the list. I advise my clients to come up with a really good reason (free sample of the service, free educational guide, subscriber only content) for people to give you their email address and create an opt in. You can then send that offer individually to people you find in your target market. Get into Facebook groups, do searches in LinkedIn, join chamber of commerces or other local business organizations where those potential clients are hanging out. Send them an email introducing yourself and give them your opt in offer along with a link to the sign up form.
Best of luck!