If your mail inbox was quiet during the last month, it could have two causes: Either nobody contacted you, or your KDEPIM made holidays!
Jokes aside, KDEPIM 4.14.0 was released with a refactored IMAP fetching code, in preparation for a planned GMail IMAP plugin. The refactoring includes a more modern way to fetch the mails from the server, which unfortunately broke with some specific IMAP servers not completely conforming to the IMAP RFCs.
The effect of this bug is that no mails are fetched from some IMAP servers when you check for new mail, despite new mail arrived. If you were using such a server, be prepared that with the coming KDEPIM 4.14.1 update, the bug is fixed, and your inbox might get flooded with accumulated mail.
For details and patch, see KDE bug 338186. Thanks to to Christian for the prompt fix, and all reporters and testers for providing valuable feedback!