Densly defined $C_{0}$-semigroup extension

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Let $(S(t))_{t \geq 0}$ be a $C_{0}$-semigroup on $H$ where $H$ is a Hilbert space. Suppose that $(S(t))_{t \geq 0}$ satisfies the following estimate on a dense subspace on $H$ $$||S(t)x||_H \leq e^{-t}||x||_H.$$ Can this estimate be extended for any $x \in H$?. Thank you.