Each of 100 students sends messages to 50 different students. What is the least number of pairs of students who send messages to each other?

$\textbf{(A)}\ 100 \qquad\textbf{(B)}\ 75 \qquad\textbf{(C)}\ 50 \qquad\textbf{(D)}\ 25 \qquad\textbf{(E)}\ \text{None}$
1. **Calculate the total number of messages sent:**
   Each of the 100 students sends messages to 50 different students. Therefore, the total number of messages sent is:
   \[
   100 \times 50 = 5000
   \]

2. **Calculate the maximum number of unique pairs of students:**
   The number of unique pairs of students that can be formed from 100 students is given by the combination formula \( \binom{100}{2} \):
   \[
   \binom{100}{2} = \frac{100 \times 99}{2} = 4950
   \]
   This represents the maximum number of unique pairs where no two students send messages to each other.

3. **Determine the number of pairs of students who send messages to each other:**
   If no two students send messages to each other, the total number of messages would be 4950. However, we have 5000 messages sent, which means there are additional messages that form pairs where students send messages to each other. The difference between the total number of messages and the maximum number of unique pairs is:
   \[
   5000 - 4950 = 50
   \]
   This difference represents the number of pairs of students who send messages to each other.

Conclusion:
\[
\boxed{50}
\]