Which email client is specifically designed for Macintosh users?

Entourage is specifically designed for Macintosh users, making it a Mac-native email client. Developed as part of the Microsoft Office suite for Mac, it integrates well with the macOS operating system, providing features that cater to Mac users’ needs, including support for Microsoft's Exchange servers, which is often utilized in business environments.

The other email clients mentioned, while they may be available on Mac, are primarily designed for other platforms. Eudora was popular in earlier days but is not specifically designed for Macs; it has largely fallen out of use. Outlook, while available for Mac, originated as a Windows application and has features tailored primarily for that environment. Thunderbird is a versatile open-source email client that works across multiple platforms, but it is not exclusive to Mac, which differentiates it from Entourage's focus on the Macintosh ecosystem.