I am running Sylpheed 2.7.0 and will install soon version 3.0. I do not recall having had an
install problem. I do have that library:
$ locate libsylpheed-plugin-0.so.0
Normally I am using Claws-mail, except when I have to use an external print command
a feature which got lost in claws-mail or I am unable to make it work unlike in previous versions
I just installed sylpheed-3.0 and to run it I had to do "ldconfig" first as sudo otherwise
I got the same error you mentioned. In fact giving command:
"ldd /usr/local/bin/sylpheed|grep found"
showed it missed even 2 libraries. Not anymore after ldconfig.