So, I'm officially giving up on kmail2 (i.e., the Akonadi-based version of kmail) on the last one of my PCs now. I have tried hard and put in a lot of effort to get it working. However, it costs me a significant amount of time and effort just to be able to receive and read e-mail - meaning hanging IMAP resources every few minutes, the feared "Multiple merge candidates" bug popping up again and again, and other surprise events. That is plainly not acceptable in the workplace, where I need to rely on e-mail as means of communication. By leaving kmail2 I seem to be following many many other people... Even dedicated KDE enthusiasts that I know have by now migrated to Trojita or Thunderbird.
My conclusion after all these years, based on my personal experience, is that the usage of Akonadi for e-mail is a failed experiment. It was a nice idea in theory, and may work fine for some people. I am certain that a lot of effort has been put into improving it, I applaud the developers of both kmail and Akonadi for their tenaciousness and vision and definitely thank them for their work. Sadly, however, if something doesn't become robust and error-tolerant after over 5 (five) years of continuous development effort, the question pops up whether the initial architectural idea wasn't a bad one in the first place - in particular in terms of unhandleable complexity.
I am not sure why precisely in my case things turn out so badly. One possible candidate is the university mail server that I'm stuck with, running Novell Groupwise. I've seen rather odd behaviour in the IMAP replies in the past there. That said, there's the robustness principle for software to consider, and even if Groupwise were to do silly things, other IMAP clients seem to get along with it fine.
Recently I've heard some rumors about a new framework called Sink (or Akonadi-Next), which seems to be currently under development... I hope it'll be less fragile, and less overcomplexified. The choice of name is not really that convincing though (where did my e-mails go again)?
Now for the question and answer session...
Question: Why do you post such negative stuff? You are only discouraging our volunteers.
Answer: Because the motto of the drowned god doesn't apply to software. What is dead should better remain dead, and not suffer continuous revival efforts while users run away and the brand is damaged. Also, I'm a volunteer myself and invest a lot of time and effort into Linux. I've been seeing the resulting fallout. It likely scared off other prospective help.
Question: Have you tried restarting Akonadi? Have you tried clearing the Akonadi cache? Have you tried starting with a fresh database?
Answer: Yes. Yes. Yes. Many times. And yes to many more things. Did I mention that I spent a lot of time with that? I'll miss the akonadiconsole window. Or maybe not.
Question: Do you think kmail2 (the Akonadi-based kmail) can be saved somehow?
Answer: Maybe. One could suggest an additional agent as replacement to the usual IMAP module. Let's call it IMAP-stupid, and mandate that it uses only a bare minimum of server features and always runs in disconnected mode... Then again, I don't know the code, and don't know if that is feasible. Also, for some people kmail2 seems to work perfectly fine.
Question: So what e-mail program will you use now?
Answer: I will use kmail. I love kmail. Precisely, I will use Pali Rohar's noakonadi fork, which is based on kdepim 4.4. It is neither perfect nor bug-free, but accesses all my e-mail accounts reliably. This is what I've been using on my home desktop all the time (never upgraded) and what I downgraded my laptop to some time ago after losing many mails.
Question: So can you recommend running this ages-old kmail1 variant?
Answer: Yes and no. Yes, because (at least in my case) it seems to get the basic job done much more reliably. Yes, because it feels a lot snappier and produces far less random surprises. No, because it is essentially unmaintained, has some bugs, and is written for KDE 4, which is slowly going away. No, because Qt5-based kmail2 has more features and does look sexier. No, because you lose the useful Akonadi integration of addressbook and calendar.
That said, here are the two bugs of kmail1 that I find most annoying right now: 1) PGP/MIME cleartext signature is broken (at random some signatures are not verified correctly and/or bad signatures are produced), and 2), only in a Qt5 / Plasma environment, attachments don't open on click anymore, but can only be saved. (Which is odd since e.g. Okular as viewer is launched but never appears on screen, and the temporary file is written but immediately disappears... need to investigate.)
Question: I have bugfixes / patches for kmail1. What should I do?
Answer: Send them!!! I'll be happy to test and forward.
Question: What will you do when Qt4 / kdelibs goes away?
Answer: Dunno. Luckily I'm involved in packaging myself. :)