https://sendpulse.com – Free upto 500 subscribers.
https://convertkit.com – Free upto 1000 subscribers.
https://mailchimp.com – Free upto 2000 subscribers.
https://www.gmass.co – Allow you to send mass email using Gmail and G Suite.
OpenEMM – Install instructions.
https://www.klaviyo.com – email marketing
https://github.com/AfterShip/email-verifier – email verifier
https://www.lemlist.com/cold-email-templates – Cold Email Templates.
https://anymailfinder.com – find email address of business owner.
https://www.apollo.io/ – Apollo is too for cold-outbound email. They have access to everyone’s email with 99% accuracy and a super simple drip sequence.