Recs.
Updated
Thunderbird is a free cross platform e-mail application that is easy to use and customize.
SpecsUpdate
Pros
Pro Large assortment of plugins
Thunderbird offers a huge amount of extensions to expand the usability and options of the client.
Pro There is a plug-in (not free) that allows full Exchange integration
This addon can be found on the Mozilla Extensions for Thunderbird.
Cons
Con With version 68 even more add-ons and all but two themes have become incompatible
Recommendations
Comments
Flagged Pros + Cons
Pro Available on Linux, OS X and Windows
The program is available on Linux, macOS and Windows. It works the same on each platform and you can even transfer your settings between platforms.
Pro Send large files easy
Install pCloud plugin for Thunderbird and your files (upto 20 GB free storage included) will be received as download links
Pro Consistency
App design has not changed significantly since its beginning, making its learning curve negligent.
Pro Consistent and involving a moderate learning curve
The app design of Mozilla Thunderbird has not changed significantly since its beginning, making its learning curve almost non-existent.
Pro Large assortment of plugins available
Mozilla Thunderbird offers a huge amount of extensions to expand the usability and options of the client.
Con Outdated interface
Thunderbird's interface is very outdated and unpleasant. It feels more like a Windows XP application than like a modern MacOS one.
Out of Date Pros + Cons
Con Interface is woefully dated
Thunderbird's interface is very outdated and unpleasant. It feels more like a Windows XP application than like a modern MacOS one.
Con Development slowed to only focus on stability and security
Thunderbird has slowed down its development of features to only focus resources on security and stability.
But still, Mozilla (the project owner) still accepts any improvements one submits as long as it follows their security standards.
Con Terrible integration with macOS
Simple things - drag/dropping pictures doesn't work, for instance.