As it stands now, claws-mail automatically depends on libSM and libICE, if they are installed; the ebuild does not take this into account. I have only looked into the current version (3.0.0) and do not know how older ones behave. The attached patch hopefully fixes this, making the dependency optional.
Created attachment 130965 [details, diff] claws-mail-3.0.0.ebuild patch for dependency on session libraries
Patch used in 3.0.0-r1. Good catch, thanks!