bool comp( const CMail & x ) const;

temp.sort( CMail::comp );

list<CMail> temp;

bool comp(const CMail& lhs, const CMail& rhs )
{
  return lhs.comp(rhs);
}

temp.sort(comp);

temp.sort([](const CMail& lhs, const CMail& rhs ){return lhs.comp(rhs);});

bool comp( const CMail & x1, const CMail& x2) {
    return 1;
}

int main(int argc, char** argv) {
    std::list<CMail> l;
    l.assign( 4, CMail());
    l.sort( &comp);
}

int main(int argc, char** argv) {
    std::list<CMail> l;
    l.assign( 4, CMail());
    l.sort( [](const CMail& x1, const CMail& x2) { return x1.comp( x2);});
                                                           ^^^^ //member function
}

#include <functional>

...

temp.sort (std::mem_fn (&CMail::comp)); // requires C++11

bool cmail_sorter (CMail const& lhs, CMail const& rhs) {
  return lhs.comp (rhs);
}

...

temp.sort (&cmail_sorter);