/usr/lib/x86_64-linux-gnu/libsylpheed-plugin-0.so.1 is in sylpheed 3.5.0~beta1~r3426-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libsylpheed-plugin-0.so.1.2.0
/usr/lib/x86_64-linux-gnu/libsylpheed-plugin-0.so.1.2.0