Find all polynomials $p$ with integer coefficients such that the number $p(a) - p(b)$ is divisible by $a + b$ for all integers $a, b$, provided that $a + b \neq 0$.

1. **Given Condition**: We need to find all polynomials \( p(x) \) with integer coefficients such that \( a + b \mid p(a) - p(b) \) for all integers \( a \) and \( b \), provided that \( a + b \neq 0 \).

2. **Initial Setup**: Let \( P(a, b) \) denote the statement \( a + b \mid p(a) - p(b) \).

3. **Symmetry Argument**: Consider the case when \( b \) is replaced by \(-b\). We have:
   \[
   a + b \mid p(a) - p(b) \quad \text{and} \quad a + (-b) \mid p(a) - p(-b)
   \]
   Since \( a + (-b) = a - b \), we get:
   \[
   a - b \mid p(a) - p(-b)
   \]

4. **Combining Results**: Subtract the two results:
   \[
   a + b \mid p(a) - p(b) \quad \text{and} \quad a - b \mid p(a) - p(-b)
   \]
   Subtracting these, we get:
   \[
   a + b \mid (p(a) - p(b)) - (p(a) - p(-b)) = p(b) - p(-b)
   \]

5. **Infinitely Many Divisors**: Fix \( b \) and vary \( a \) over all integers. Since \( a + b \) can take infinitely many values, \( p(b) - p(-b) \) must be divisible by infinitely many integers. The only integer with infinitely many divisors is zero. Therefore:
   \[
   p(b) - p(-b) = 0 \implies p(b) = p(-b)
   \]
   This implies that \( p(x) \) is an even function.

6. **Form of \( p(x) \)**: Since \( p(x) \) is an even function, it can be written as:
   \[
   p(x) = \sum_{i=0}^{n} a_i x^{2i}
   \]
   where \( a_i \) are integer coefficients.

7. **Verification**: We need to verify that this form satisfies the original condition. Consider:
   \[
   p(a) - p(b) = \sum_{i=0}^{n} a_i (a^{2i} - b^{2i})
   \]
   Notice that \( a^{2i} - b^{2i} \) can be factored as:
   \[
   a^{2i} - b^{2i} = (a^i - b^i)(a^i + b^i)
   \]
   Therefore:
   \[
   p(a) - p(b) = \sum_{i=0}^{n} a_i (a^i - b^i)(a^i + b^i)
   \]
   Since \( a + b \mid a^i + b^i \) for all \( i \), it follows that \( a + b \mid p(a) - p(b) \).

Conclusion:
\[
\boxed{p(x) = \sum_{i=0}^{n} a_i x^{2i}}
\]