Haemers's Laplacian eigenvalue conjecture for graph toughness

Let $G$ be a simple graph with minimum degree $\delta$. Let $\mu_2$ and $\mu_n$ denote the second-smallest and largest eigenvalues of the Laplacian matrix of $G$, respectively, and let $t(G)$ be its toughness. Haemers's conjecture.

$$t(G)\geq\frac{\mu_2}{\mu_n-\delta}.$$

The source attributes this conjecture to Haemers and presents it as a proposed lower bound for the toughness of arbitrary graphs in terms of Laplacian eigenvalues. The supplied material does not state whether it has been resolved.