Missive

Without a doubt, some of the best collaboration tools out there can make the team more productive. One of the most important, and often overlooked, aspects of any collaboration app is, of course, user-accessibility which is often sacrificed for as many features as possible.
This is, by no means, the case of Missive, a very user-orientated email app for Windows and macOS that boasts an impressive mix of collaboration, email and chat tools.
To get started, simply log in by using an official Missive account or with the help of your Google credentials.

User-friendly and modern GUI wrapped around powerful email and chat tools

Even from the get-go, things should feel quite familiar since Missive sports a typical layout found on most email clients out there, with the main window split into three main sections.
One of the main highlights of this app, besides its unified Inbox capabilities, is the fact that it allows you and your team members to compose emails together. Another interesting aspect is that it also supports shared conversations, which means that you can discuss with other team members directly while reviewing or composing emails.

Unified collaborative environment with practical email and chat tools

There are other noteworthy bits and bobs, as well. For example, you can comment directly within email threads, assign tasks to team members and organize conversations for the whole department or only for specific coworkers.
Quite conveniently, both chat and emails can be managed in a very similar fashion. For instance, you are able to archive and snooze both emails and chats, draft emails and even schedule them for a time of your choosing. The app also provides you with a self-explanatory "forgotten attachment reminder." and, as expected, templates and comprehensive text and HTML signatures are also supported.

Improve your workflow with the help of this Swiss Army Knife-like email client

Unfortunately, for the time being, Missive only comes with support for Gmail and Google Apps. That said, if your company's email and workflow structure are based on the services provided by Google, Missive is definitely a must-have.
To sum it up, Missive is one of the best full-featured email clients that efficiently integrates both chat and collaboration tools, without losing valuable points in the user-accessibility department.