Right, it isn't an isolated problem. I have to proof one quite big theorem and a part of this proof is this inequality.
I have another idea, how could this theorem be proved.
We have to proof, that $ \frac{m!}{\lambda^{m+1}} \cdot \sum_{j=0}^{m} \frac{\lambda^j}{j!} \leq \frac{2}{\sqrt...