This is the official repo of Mailsync. Mailsync syncs user data directly from MySQL and PostgreSQL to Mailchimp and Campaign Monitor.
You can use it under the MIT License, which pretty much says you can use it freely for commercial and non-commercial projects as long as you provide attribution. More on this here.
What is Mailsync here.
If you need information on how to install or configure it - here.
Small FAQ which includes general questions - here.
Issues here.