In general, we do not have any out-of-the-box functionality to integrate Piano with a third-party mailing service. Alternatively, you can take a look at the integration with our own email service provider - ESP - here. More information about this product can be shared by your Piano Account Representative.
For Composer-only clients, it's possible to configure a newsletter sign-up template with input fields, that gathered user emails, which were then sent from our templates to the 3rd party software. More information about this is available here.
If you're using Mailchimp, you can follow the instructions here to send data from Piano to this platform.
In addition, if you'd like to not show the Newsletter sign-up template to users that have already signed up for the newsletter, you would need to create a Composer experience including a Show template card with this template and a Set cookie card, where you would set a custom cookie for the on-click event (which is triggered for those users who clicked on "Sign up" button in the template). Afterward, you can target only users without this custom cookie.