(The actual version I'm using is 4.14.5, from the official Arch Linux repositories, but as it was unavailable in the version list I chose the closest stable version.) I'm unsure if this is a problem in KMail or Akonadi, but since KMail is where the symtpoms manifest themselves, and I know next to nothing about how the interaction between KMail and Akonadi works, I'm reporting this here to start off. I have KMail configured to sync with two IMAP accounts, one being a Gmail account, set up using Google Apps, and the other being some kind of Outlook Web App(?) account. The Gmail account has three subfolders to the Inbox folder. I am able to fetch and read messages from the Inbox folder as well as two of the subfolders, but whenever I try to view the third subfolder KMail becomes unable to fetch messages, including messages from other folders. Reproducible: Always Steps to Reproduce: 1. Quit KMail 2. Restart Akonadi (akonadictl restart) 3. Start KMail 4. Switch to the folder in question Actual Results: KMail stalls at "Retrieving Folder Contents - Please wait" indefinitely, and is unable to fetch messages from any other folder or even other accounts. In the terminal I see the following output from Akonadi when switching to the problematic folder: posting retrieval request for item 41750 there are 1 queues and 0 items in mine request for item 41750 still pending - waiting processing retrieval request for item 41750 parts: ("RFC822") of resource: "akonadi_imap_resource_2" At this point the progress bar starts counting up from 0% to 100% for about 10 seconds. Once the progress bar reaches 100%, it immediately jumps back to 96%. At the same time I see output along the following lines from Akonadi, I think this might be related to the message KMail was trying to fetch: 40616 "3041" "<91AFCD21.4365920@gmail.com>" 40630 "3041" "<91AFCD21.4365920@gmail.com>" Meanwhile KMail is displaying "Retrieving Folder Contents - Please wait" indefinitely. This does not go away if I try to switch to another folder, even if the folder belongs to my other account. After several minutes I get the following error message: request for item 41750 "3206" failed: "Unable to retrieve item from resource: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken." ItemRetrieverException : Unable to retrieve item from resource: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken. KMail continues to be unable to fetch/display messages however. Expected Results: KMail should fetch a message from the folder and display it, and afterwards allow me to fetch other messages.
Update: After experimenting some more I found that the issue occurs when I synchronize the folder in general, not only when I actually select it. I was able to work around the issue by changing the folder properties to never sync, and I was thus able to at least read the messages already listed in the folder. I then did a Google search for the gmail address that was printed by Akonadi (the actual address differs slightly from what I wrote above), and found a reference to a specific e-mail to a mailing list I subscribe to. I looked up the message in question in KMail and found that I for some reason had two copies of the same message. I tried simply removing both entries, and after doing so I can now synchronize the folder properly. I am leaving this issue unresolved, however, as I still consider it a bug that KMail stops functioning like this simply because something went wrong with a single e-mail message. It is also likely that another user in the situation would not have been able to resolve this on their own. I have no idea how I could have resolved this issue had I not been lucky enough to find the address outputted by Akonadi on Google.
This bug has never been confirmed for a KDE PIM version that is based on KDE Frameworks (5.x). Those versions differ significantly from the old 4.x series. Therefore, I plan to close it in around two or three months. In the meantime, it is set to WAITINGFORINFO to give reporters the oportunity to check if it is still valid. As soon as someone confirms it for a recent version (at least 5.1, ideally even more recent), I'll gladly reopen it. Please understand that we lack the manpower to triage bugs reported for versions almost two years beyond their end of life.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.1 aka 15.12, preferably more recent), please open a new one unless it already exists. Thank you for all your input.