Learn Spanish (Spain): Él le envía una carta a ella.

In Spanish, the indirect object pronoun le is required, but it can mean "to him", "to her", or "to you (formal)". Because it's ambiguous, we often add a ella to clarify exactly who is receiving the letter.

Can I just drop le and say "Él envía una carta a ella"?

No, that sounds unnatural in Spanish. You almost always need the indirect object pronoun (le), even when you explicitly state the receiver with a phrase like a ella. Think of le as the mandatory piece, and a ella as an optional clarifier.

Where does le go if the sentence is negative, like "He doesn't send her a letter"?

The word no always goes right before the pronoun. You would say, "Él no le envía una carta a ella". The pronoun le likes to stay glued to the front of the conjugated verb.

Why does envía have an accent mark?

The infinitive is enviar (to send). Normally, the vowels "i" and "a" blend into a single syllable. The accent mark on envía breaks that blend, making you pronounce the "i" and "a" as two separate syllables and putting the verbal stress heavily on the "i".