Properties of series of exponentials whose exponents satisfy to a condition of Levinson type

Let sequence $\{\lambda_n\}$, $0<\lambda_n\uparrow\infty$, satisfies to a condition of Levinson type. It is obtained exact estimate of growth of the Dirichlet series $f(z)=\sum_{n=1}^\infty a_ne^{\lambda_nz}$ on the curve of bounded slope, depending only on its coefficients and exponents.