Poste.io is described as 'Full featured email server built in docker with free plan' and is a Mail Server in the Online Services category. There are more than 10 alternatives to Poste.io for a variety of platforms, including Windows, Self-Hosted solutions, Linux, Online / Web-based and BSD. The best alternative is Zimbra Collaboration Suite, which is both free and Open Source. Other great apps like Poste.io are mail.com (Free), Mail-in-a-box (Free, Open Source), hMail Server (Free, Open Source) and iRedMail (Freemium, Open Source).
Xeams is a secure and powerful mail server for that includes a powerful spam filtering engine that eliminates up to 99% of junk email right out of the box. The software offers a flexible approach to email, making it a very user friendly server.