Kmail is on screen for just long enough to be seen before it suddenly closes leaving this message on the command line: > symbol lookup error: /usr/lib/x86_64-linux-gnu/libKF5MessageCore.so.5: undefined symbol: _ZNK5KMime5Types7Mailbox13prettyAddressEv
it's a problem with your distro. Please report this bug to neon. Regards
Assuming it is fixed in current neon.