  Let $\mathcal{X}$ be a separable Hilbert space with norm $\|\cdot\|$ and let
$T>0$. Let $Q$ be a linear, self-adjoint, positive, trace class operator on
$\mathcal{X}$, let $F:\mathcal{X}\rightarrow \mathcal{X}$ be a (smooth enough)
function and let $W(t)$ be a $\mathcal{X}$-valued cylindrical Wiener process.
For $\alpha\in [0,1/2]$ we consider the operator
$A:=-(1/2)Q^{2\alpha-1}:Q^{1-2\alpha}(\mathcal{X})\subseteq
\mathcal{X}\rightarrow \mathcal{X}$. We are interested in the mild solution
$X(t,x)$ of the semilinear stochastic partial differential equation
\begin{gather*} \left\{\begin{array}{ll} dX(t,x)=\big(AX(t,x)+F(X(t,x))\big)dt+
Q^{\alpha}dW(t), & t\in(0,T];\\ X(0,x)=x\in \mathcal{X}, \end{array}\right.
\end{gather*} and in its associated transition semigroup \begin{align*}
P(t)\varphi(x):=\mathbb{E}[\varphi(X(t,x))], \qquad \varphi\in
B_b(\mathcal{X}),\ t\in[0,T],\ x\in \mathcal{X}; \end{align*} where
$B_b(\mathcal{X})$ is the space of the bounded and Borel measurable functions.
We will show that under suitable hypotheses on $Q$ and $F$, $P(t)$ enjoys
regularizing properties, along a continuously embedded subspace of
$\mathcal{X}$. More precisely there exists $K:=K(F,T)>0$ such that for every
$\varphi\in B_b(\mathcal{X})$, $x\in \mathcal{X}$, $t\in(0,T]$ and $h\in
Q^\alpha(\mathcal{X})$ it holds \[|P(t)\varphi(x+h)-P(t)\varphi(x)|\leq
Kt^{-1/2}\|Q^{-\alpha}h\|.\]
