DeliverySlip is described as 'The way you communicate is about to change without having to change your tools or email address. Send, sign, track and manage your email from your inbox (Gmail, O365, Outlook, etc…)' and is an app in the Security & Privacy category. There are nine alternatives to DeliverySlip for a variety of platforms, including Online / Web-based, iPhone, iPad, Android and Android Wear. The best alternative is ProtonMail, which is both free and Open Source. Other great apps like DeliverySlip are Yandex.Mail (Free), Hushmail (Paid), Tildamail (Free) and Pony (Free).
Hushmail is a commercial email service focusing on security with features such as industry-standard OpenPGP encryption and two-step verification. All communications also use a secure SSL/TLS connection.
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.
Virtru provides digital privacy technology to let you easily share information securely through all major email programs and applications. Virtru puts you in charge of how your digital information is shared.