YunoHost is a free and open-source self-hosting platform from France, built on Debian, offering one-click deployment of 400+ apps and built-in mail services from a single server with 2GB of RAM or more.
CentOS Web Panel (now Control Web Panel) is a free open-source server control panel running on 10,000+ dedicated servers and VPS, with an Apache/Nginx/mail stack and Mod Security protection for small teams managing CentOS/AlmaLinux hosts.
MXToolbox is a US-based online email and DNS diagnostic platform founded in 2003, offering 200+ free diagnostic tools including DNS lookup, blacklist check, SMTP test, and domain health monitoring.
ISPConfig is an open-source Linux hosting control panel created in 2005 by German developers Till Brehm and others, released free under the BSD license. It offers multi-server management with web, DNS, mail, and database administration in one console, a cost-effective open alternative to cPanel and Plesk.