I have always liked Geary email app for the uncluttered user interface. Geary is elegantly simple. In a sense, it reminds me of Apple Mail, which I really like. However, Geary is presently experiencing issues with displaying three panes on systems that have small screens. The issue seemingly started with the upgrade of a key library. So that has forced me to go in search of a new email client.
In the past, I used Thunderbird but I think that the user interface needs some improvement. For example, one of the things I appreciate in Geary is that in the pane that shows the list of mails, there is a multi-line view that contains the name of the sender, subject of the email, and a brief snippet of the mail. The effect of this is that the pane can be compact and this gives more space for the reading pane to be expanded. Thunderbird, on the other hand, has separate columns for the name of the sender and the subject of the email. The developers have made it clear that the multi-line functionality will be implemented, but that they're focused on other things.
Again, I went searching for a solution and found Betterbird email app, described as:
Betterbird is a fine-tuned version of Mozilla Thunderbird, Thunderbird on steroids, if you will. Betterbird is better than Thunderbird in three ways: It contains new features exclusive to Betterbird, it contains bug fixes exclusive to Betterbird and it contains fixes that Thunderbird may ship at a later stage. Please refer to this feature table for examples. This should give you an impression of where the project is headed.
Betterbird has some features that I like, although like Thunderbird, I find the user interface to be too cluttered.
So if you're searching for an improved Thunderbird client, perhaps you can give Betterbird a try. Just run over to betterbird dot eu and check it out.