Find all integer polynomials $f$ such that there are infinitely many pairs of relatively prime natural numbers $(a,b)$ so that $a+b  \mid f(a)+f(b)$.
1. **Decompose \( f(x) \) into odd and even parts:**
   Let \( f(x) = g(x) + h(x) \), where \( g(x) \) is an odd function and \( h(x) \) is an even function. This decomposition is possible because any polynomial can be written as the sum of an odd and an even polynomial.

2. **Condition for \( g(x) \):**
   Since \( g(x) \) is an odd function, we have \( g(-x) = -g(x) \). We need \( a + b \mid g(a) + g(b) \) for all pairs \((a, b)\). For \( g(x) \) to satisfy this condition, it must be that \( g(a) + g(b) \) is divisible by \( a + b \).

3. **Condition for \( h(x) \):**
   Since \( h(x) \) is an even function, we have \( h(-x) = h(x) \). We need \( a + b \mid h(a) + h(b) \) for infinitely many pairs \((a, b)\) with \((a, b) = 1\). Note that \( a + b \mid h(a) - h(b) \), so we need \( a + b \mid 2h(b) \).

4. **Analyzing \( h(x) \):**
   Suppose \( h(x) = cx^{2n} \). Then we need \( a + b \mid 2cb^{2n} \). Since \((a + b, b^{2n}) = 1\), we must have \( a + b \mid 2c \). This implies that there are only finitely many pairs \((a, b)\) satisfying this condition, which contradicts the requirement of having infinitely many pairs.

5. **General form of \( h(x) \):**
   Suppose \( h(x) = cx^{2n} + T(x) \), where \( T(x) \) is a nonzero even function with all terms of degree higher than \( 2n \). We need \( a + b \mid 2(cb^{2n} + T(b)) \), or equivalently \( a + b \mid 2(c + \frac{T(b)}{b^{2n}}) \).

6. **Choosing \( a \) and \( b \):**
   Take \( b \) coprime with \( c \), and choose \( a = 2(c + \frac{T(b)}{b^{2n}}) - b \). This choice ensures that \( a \) is coprime with \( b \). This construction shows that there are infinitely many pairs \((a, b)\) satisfying the condition.

7. **Conclusion:**
   The function \( f(x) \) must be such that it has no or more than two even terms. This ensures that the condition \( a + b \mid f(a) + f(b) \) is satisfied for infinitely many pairs of relatively prime natural numbers \((a, b)\).

The final answer is \( \boxed{ f(x) } \) must be a polynomial with no or more than two even terms.