Be open about it. Copy this reply, fill in the brackets and send it from your own address:
Hi [first name],
I found your business contact details through GetProspect (getprospect.com), a B2B database of professionals. GetProspect collects profile information such as name, job title and employer from publicly available professional sources and licensed B2B data providers. Your business email address was generated from your name and your company's email format, and then verified.
I contacted you because [why your message is relevant to their role].
If you would rather not hear from me, just reply and I will delete your details from my records. To have your data removed from GetProspect as well, visit https://getprospect.com/personal-data.
[Your name, company and contact details]
If they ask you to stop, delete them from your lists, CRM and sending tools, and do not contact them again.
This is general information, not legal advice.