I was typing a reply to an email, then I temporarily switched to another window and when I switched back kmail was no longer responsive. The kmail process was constantly consuming 100% of a CPU core and 2.7 GB of RAM - that is compared to around 180 MB that it consumes right after start. I have two IMAP email accounts attached, one of the akonadi_imap_resource processes was consuming 1.7 GB of RAM during the incident, but was mostly idle on CPU. The memory consumption did not go down after I restarted kmail. I got a few stacktraces from the kmail process as it was hanging (attached). I'm not sure what was the trigger for this behavior. This happened only once so far. Reproducible: Sometimes Kubuntu 15.04, x86_64. KDE Frameworks and SC installed from Kubuntu backports.
Created attachment 93036 [details] kmail backtrace 1
Created attachment 93037 [details] kmail backtrace 2
Created attachment 93038 [details] kmail backtrace 3
Created attachment 93039 [details] kmail backtrace 4
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.