MemberPress ConvertKit GPL Overview:

ConvertKit is an advanced email marketing service that caters to bloggers. It includes email capture forms, broadcasts, beautiful templates, autoresponders, tagging, and advanced email automation workflows.

ConvertKit is designed for creators who don’t want to spend their time messing with the technical details. Use it to quickly build landing pages to share your ideas as they come, and write engaging emails that sell your products by connecting with your audience.