OnionMail is described as 'anonymous and encrypted mail server designed to run on the TOR network without losing the ability to communicate with the Internet. This ensures greater privacy to the users but also protect against eavesdropping of the NSA and others' and is an app in the security & privacy category. There are more than 10 alternatives to OnionMail for a variety of platforms, including Online / Web-based, Linux, Mac, Windows and Android. The best alternative is ProtonMail, which is both free and Open Source. Other great apps like OnionMail are Tutanota, Vivaldi Mail, Yandex.Mail and Enigmail.
OnionMail is mainly a WebMail Provider but alternatives to it may also be Calendar Apps or Email Clients. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of OnionMail.
Tildamail is a free, decentralized, and secure email system, developed by Stanford graduates. We are an end-to-end encrypted email system that protects your privacy. Tilda is a standard for DApps backed by self-sovereign data. User data never touches our servers.