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.
Assuming it is fixed in current neon.