postale.io is described as 'Free domain email service. Simple, safe, fast, 7/7 free support, and all the essential features a mail service should have' and is an app. There are more than 10 alternatives to postale.io for a variety of platforms, including Online / Web-based, Android, iPhone, iPad and Linux. The best alternative is ProtonMail, which is both free and Open Source. Other great apps like postale.io are Gmail, Tutanota, Outlook.com and Zoho Mail.
postale.io is mainly a WebMail Provider but alternatives to it may also be Email Clients or Calendar Apps. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of postale.io.