$(x_{n})_{-\infty<n<\infty}$ is a sequence of real numbers which satisfies $x_{n+1}=\frac{x_{n}^2+10}{7}$ for every $n \in \mathbb{Z}$. If there exist a real upperbound for this sequence, find all the values $x_{0}$ can take.
1. **Non-negativity of the sequence:**
   Given the recurrence relation \( x_{n+1} = \frac{x_n^2 + 10}{7} \), we start by noting that if \( x_n \geq 0 \), then \( x_{n+1} \geq \frac{10}{7} > 0 \). Therefore, if \( x_0 \) is non-negative, all subsequent terms \( x_n \) will also be non-negative. 

2. **Definition of the function \( f \):**
   Define the function \( f: \mathbb{R} \to \mathbb{R} \) by \( f(x) = \frac{x^2 + 10}{7} \). The recurrence relation can be written as \( x_{n+1} = f(x_n) \).

3. **Fixed points of \( f \):**
   To find the fixed points of \( f \), solve \( f(x) = x \):
   \[
   \frac{x^2 + 10}{7} = x \implies x^2 + 10 = 7x \implies x^2 - 7x + 10 = 0.
   \]
   Solving this quadratic equation, we get:
   \[
   x = \frac{7 \pm \sqrt{49 - 40}}{2} = \frac{7 \pm 3}{2} \implies x = 5 \text{ or } x = 2.
   \]
   Thus, the fixed points are \( x = 2 \) and \( x = 5 \).

4. **Behavior of the sequence:**
   - For \( x_n > 5 \), \( f(x_n) > x_n \), so the sequence is increasing and unbounded.
   - For \( x_n = 5 \), the sequence is constant.
   - For \( 2 < x_n < 5 \), \( f(x_n) < x_n \), so the sequence is decreasing and converges to 2.
   - For \( x_n = 2 \), the sequence is constant.
   - For \( 0 < x_n < 2 \), \( f(x_n) > x_n \), so the sequence is increasing and converges to 2.

5. **Boundedness of the sequence:**
   Since the sequence is bounded, \( x_n \) must lie within the interval \([0, 5]\). However, no term \( x_n \) can be strictly less than 2, because if \( x_n < 2 \), the sequence will eventually increase to 2, contradicting the boundedness condition.

6. **Possible values for \( x_0 \):**
   - \( x_0 = 2 \) is possible if the sequence is constant at 2.
   - \( x_0 = 5 \) is possible if the sequence is constant at 5.
   - Any value \( x_0 \) in the interval \( (2, 5) \) is possible, as the sequence will decrease to 2.

Therefore, the range of possible values for \( x_0 \) is the interval \([2, 5]\).

The final answer is \( \boxed{ [2, 5] } \).