I've been using Claws Mail for longer than I can remember. It's a very capable email client that's actively developed, and it does everything I want an email client to do. The developers are also approachable (via IRC) and very helpful. Yet, after I upgraded my main PC to Fedora 32 I decided to give Evolution a try.
There's no real reason for the switch. My only gripe with Claws is that the interface is ugly. It's the one application on my PC that doesn't have all the GNOME prettiness. But then, there isn't a GNOME email client that's both feature-rich and pretty. What really triggered the switch was that I wanted to reorganise my emails, and it seemed a good time to try something else at the same time.
As for reorganising my emails, I had over 100 archive folders, often several directories deep. I had separate folders for different email accounts, and within those folders I had subfolders for things like contacts, clients, projects and whatever else deserved its own folder. The logic behind that approach is that it's easy to find stuff. If I needed to check my correspondence with a client then I could find everything without doing a search. The downside of using lots of folder is, well, that you end up with a very large number of folders.
All received emails now live in the inbox under "On This Computer" and all sent emails are in the Sent folder. I haven't added any custom folders, and so far I'm happy with the set-up. The reality is that I rarely need to find an old email, and when I do I can use Evolution's search feature. Finding the right search query can be a little tricky, so much so that I prefer to grep the data folder on the file system. The search feature seems okayish though. As long a search is fairly specific it will find what I'm looking for.
Things I miss
There a couple of things I miss. The main one is that Claws is very smart when it comes to plain text. Of course, it better be: plain text is all it does. Still, it's impressive. Incoming HTML emails usually show the text without any of the HTML and you can use basic markdown when you compose emails. The latter is particularly nice when want to include things such as bulleted lists. You can simply start a line with an asterix to use bullets, and bullet points that wrap to a new line are indented correctly. Evolution is terrible at displaying HTML emails as plain text (it just prints all the HTML code) and it doesn't have any fancy features when you compose an email in plain text. It's very much an email client that has accepted that people send each other web pages via email.
To be fair, that's a sensible approach. During my time as Web Master I occassionally had to integrate Mail Chimp newsletter sign-up forms. That involved adding myself to the mailing list, and I always opted to receive newsletters in plain text. I also always made sure that sign-up forms included the option to receive newsletters in plain text. That was really a waste of time as nobody ever selected that option. The reality is that very few people care about plain text email. I would love to see proper plain text support in other email clients but I understand why that feature is at the very bottom of the to-do list.
Another thing I miss are tags. I never used tags much in Claws but they are a nice feature. Put simply, you can give emails one or more arbitrary tags, and you can use them when searching for emails. As long as you consistently apply and organise your tags it can be an alternative for using an insane amount of (sub)foldes to organise emails. The reason I didn't go that route is that applying tags retrospectively was too time-consuming.
Evolution does let you apply labels but the feature is very limited. Each label needs to have its own colour, and when you apply a label the email will be displayed in the message pane using that colour. It quickly turns your message pane in a festival of colour, and of course it implies that a single email can only have a single label. In short, labels in Evolution can be useful to mark one or two emails as, say, "important". However, labels can't be used to categorise emails in a meaningful way.
Things I like
The main thing I like about Evolution is that it does more than just email: it also comes with a calendar, tasks manager and memos. Over the years I've tried various solutions for calendars and tasks but none of them have stuck. The main problem is really one of stamina. After a while I stop looking at a calendar or task list, and before you know it everything is hopelessly out of date. I'm hoping Evolution will keep me properly organised. I usually have my email client open, and the calendar and task list are accessible with a single click. I might be a little optimistic here, as opening a stand-alone application isn't much of an effort. So far I've managed to only work on projects that are on my task list though.
What is disappointing is that the Evolution Flatpak doesn't integrate with GNOME's notifications widget. I do get a notification when I receive a new email but it's not listed in the widget. Calendar appointments and tasks don't appear in the notifications area either. Apparantly, this is a feature rather than a bug.
Other than that I like the look of Evolution. It doesn't adhere to GNOME's Human Interface Guidelines but it looks much cleaner than Claws.
Things I hate
My main problem with Evolution so far is that it sometimes stuggles to retreive new emails. This consitently happens when I boot or unsuspend my PC, and it happens at random times as well. It may take Evolution anywhere between one and two minutes to get new emails. I've seen this issue every time I've installed Evolution. In fact, this was an issue in Ubuntu 10.04 - I was quite happy that Ubuntu replaced Evolution with Thunderbird in its next major release.
If I'm honest, I think the switch was a mistake. At some point using a text-only email client will be too hard-core, but I don't think that time has arrived just yet.