When comparing Mozilla Thunderbird vs kmail, the Slant community recommends Mozilla Thunderbird for most people. In the question“What are the best Linux email clients?” Mozilla Thunderbird is ranked 3rd while kmail is ranked 8th.
Ranked in these QuestionsQuestion Ranking
Pro Free (as in freedom and beer)
Pro Good filtering system
Mozilla Thunderbird offers a flexible filtering system with the ability to set flags and read/unread, as well as sort/assign to new mail directories. There are also numerous plugins available to assist in the filtering.
Pro Very reliable
Version after version, Mozilla Thunderbird works as expected.
Pro Lightning Calendar and Address Book integration
Pro Large assortment of plugins available
Mozilla Thunderbird offers a huge amount of extensions to expand the usability and options of the client.
Pro Available on Linux, OS X and Windows
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 Great flexibility
More options and extended settings than most power users could dream of. Get it working how YOU want.
Pro Tabs for navigation
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 Proper kde intergation
Its the only graphical email client which is properly integrated into kde plasma.
Pro Has system tray icon
Good integration with plasma 5
Pro Supports POP3 and IMAP mailboxes
Con Terrible integration with macOS
Simple things - drag/dropping pictures doesn't work, for instance.
Con Outdated interface
Thunderbird's interface is very outdated and unpleasant.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.
Con Serious bugs
Clicking on a subject line can bring up the wrong email.
Con Development slowed to only focus on stability and security
Mozilla Thunderbird has slowed down its development of features to only focus resources on security and stability.
Con Subject lines can (temporarily) disappear from the list
This is sporadic behavior.
Con Buggy related kdepim applications
kmail by itself isn't the only application which is buggy, the many of the kdepim applications are buggy as of spring 2020.
Con Poor UX
The user experience of kmail is extremely poor comparison to Thunderbird or evolution on Linux.
Con No google support
You cant pass google 'Authentication' for the majority of the kdepim applications, The only support which works is via the kde 'online accounts' under 'system settings'. This only supports youtube and google hangouts integration.
Con Buggy as of 2020
There are many bugs which are hard to track down in kmail, sometimes without any useful logs for troubleshooting. Sometimes the bugs are related to supportive applications like Akonadi, ect.
Akonadi needs lot of ram