kmail relies on libsasl2-modules for all auth needs (except oauth, for which there is a custom plugin). without the modules installed kmail is basically broken for all intents and purposes (well, technically akonadi is I suppose).

currently you can actually install kmail/kontact without it dragging in libsasl2-modules. get in touch with kdepim upstream and figure out which piece of software should hard-depend on libsasl2-modules! dvratil knows a thing or two about sasl IIRC.

impact of this is somewhat lessened because neon-desktop hard depends the modules (I think because of ubuntu platform seed). so to hit this problem one needs to either bootstrap or uninstall neon-desktop and libsasl2-modules before installing kmail.