There are a few Slackware software repositories on the internet:

• http://www.slacky.eu
• http://rlworkman.net/pkgs/
• http://slackbuilds.org/
• http://www.linuxpackages.net/

Note that the linuxpackages.net site appears as if it is no longer being actively maintained.  Regardless, if you wish to run Sylpheed-Claws on your Slackware system you’ll need a couple of Slackware packages:

• The core Claws package(s)
• The libetpan library

Thats it.  Simply install the packages with the Slackware installer (installpkg).