Sendinblue is the more common of the two in our index: it runs on 1,142 websites against 193 for Mautic, and 4 sites use both. Counts come from real scans, not vendor claims.
Mautic is a free and open-source marketing automation tool for Content Management, Social Media, Email Marketing, and can be used for the integration of social networks, campaign management, forms, questionnaires, reports, etc.
Sendinblue is an email marketing solution for small and medium-sized businesses that want to send and automate email marketing campaigns.
4 sites run both Mautic and Sendinblue. That is 2% of the sites using Mautic, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.