If you cancel it it reappears, and upon every new connection attempt a new one is opened, potentially leading to a boatload of dialogs. https://bugs.kde.org/show_bug.cgi?id=233628 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=745556
I run into this too. I suspect the developers didn't think that someone actually cares about security and cancels if the certificate is incorrect. :) Same for Konqueror.
This bug has only been reported for versions older than KDEPIM 4.14 (at most akonadi-1.3). Can anyone tell if this bug still present? If noone confirms this bug for a recent version of akonadi (part of KDE Applications 15.08 or later), it gets closed in about three months.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.0 aka 15.08), please open a new one unless it already exists. Thank you for all your input.