Important note: You'll have to have have gpgme [url]http://www.gnupg.org/(en)/related_software/gpgme/index.html[/url] compiled before you ./configure sylpheed.
When I attempted to compile gpgme it threw an error indicating it couldn't find the library libgpg-error, so I had to download and compile libgpg-error [url]http://www.gnupg.org/(en)/related_software/libgpg-error/index.html[/url].
Seems to be working so far. I like Sylpheed (it's fast) and with the addition of GPG I like it even more! And the new install didn't wipe out my inbox.