Mac native Mail App is, undoubtedly, one of the easiest mail client to configure and use, but one downside which always compels me to consider third-party email clients is that each window gets opened separately, eventually cluttering up whole desktop area (as shown in the screenshot above). MailTabs is small plug-in for Mac Mail App which brings tabbed interface. With MDI based UI, you can view and manage multiple windows under one place without having to manually adjust them at different corners of the screen. MailTabs provides the best way to organize different message windows. You can compose new message, reply to emails, forward messages and do anything related with emails under tabs. According to the developer, it’s an ability to save the sessions, so they can be restored back on launching Mail app.
It doesn’t require you to configure any sort of settings nor you have to set tabs arrangement. Before installing the plug-in, make sure that Mail App is not running in the background. Once installed, launch the Mail App, you will see a tab bar underneath the toolbars. If for some reason, tabs are not displaying in a proper way, you have to restart the app to make newly integrated tab bar adjust properly. The developer seems very keen about updating MailTabs in near future with extra keyboard shortcuts.
It works on Mac 10.5.8 and later versions.