Yesware for Gmail is available for current versions of Chrome and Firefox web browsers on any operating system.
Google Chrome stopped releasing updates for the Mac OSX 10.5 operating system with Chrome version 21 being the most recent available version. Due to this limitation, Yesware is not compatible with Mac OSX 10.5 or earlier.
Yesware for Outlook is available on Windows 7 or later using Outlook 2010, 2013, or 2016.
If you have an O365 Subscription there is a Yesware add-in available via the O365 Store within Outlook that is compatible with Outlook 2016 for Mac, and the Outlook Web Portal.