GDPR & Email Marketing
Yes, you can email your existing customers about a new service you are offering. Approaching your customer for anything else then to serve the existing services they have contracted with you is in fact a new processing activity.
Therefore, make sure that you include a link in the email for your customers to be able to opt-out. They in fact never signed up to anything else then the original services, and some customers will indeed want to opt out of this new usage of their personal data.