The best alternatives to Mail by Apple are Polymail , Spark, and Airmail. If these 3 options don't work for you, we've listed a few more alternatives below.
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Polymail is the email platform for teams. By combining email with real time collaboration and integrating it with your team's other tools, Polymail allows everyone to work together seamlessly, move faster, and be more productive.
Spark helps you take your inbox under control. Instantly see what’s important and quickly clean up the rest. Spark for Teams allows you to create, discuss, and share email with your colleagues.