According to the description of the package, this library is just backport of Ruby 1.9's features to Ruby 1.8. Since Ruby 1.9.3 are going to be default for F17/Rawhide, this package should not be required anymore. Please retire the package. Thank you.
The rubygem-tlsmail package is now retired for Fedora 17 onward. https://fedorahosted.org/rel-eng/ticket/5061