  Bounds on the exponential decay of generalized eigenfunctions of bounded and
unbounded selfadjoint Jacobi matrices are established. Two cases are considered
separately: (i) the case in which the spectral parameter lies in a general gap
of the spectrum of the Jacobi matrix and (ii) the case of a lower semi-bounded
Jacobi matrix with values of the spectral parameter below the spectrum. It is
demonstrated by examples that both results are sharp.
  We apply these results to obtain a "many barriers-type" criterion for the
existence of square-summable generalized eigenfunctions of an unbounded Jacobi
matrix at almost every value of the spectral parameter in suitable open sets.
As an application, we provide examples of unbounded Jacobi matrices with a
spectral mobility edge.
