Thunderbird is described as 'Cross-platform email client providing POP/IMAP support, robust filtering, multiple accounts, and chat. Extendable via add-ons with RSS/Atom, etc' and is a leading Email Client in the office & productivity category. There are more than 50 alternatives to Thunderbird for a variety of platforms, including Windows, Mac, Linux, Android and iPhone apps. The best Thunderbird alternative is K-9 Mail, which is both free and Open Source. Other great apps like Thunderbird are Claws Mail, Evolution, Betterbird and SeaMonkey.
Thunderbird connects with servers that use standard protocols and with servers for which a plugin was developed. ElectronMail works with ProtonMail ONLY and was specifically designed for this provider. Tutanota support is deprecated since July 2019.