There is no way to install mail-client/claws-mail-notification with an unstable version of mail-client/claws-mail, since the former requires mail-client/claws-mail-3.9.0. I suggest to make the dependency less restrictive: RDEPEND=">=mail-client/claws-mail-3.9.0 ... or RDEPEND="(=mail-client/claws-mail-3.*) ... or to add an unstable package for unstable claws-mail.
This plugin cannot be used with >=mail-client/claws-mail-3.9.1 Please enable the "notification" USE flag in later versions of claws-mail.
Thanks