In English, the difference comes down to verb tense and what you want to express:
“Aquí le envío las respuestas” means “Here I am sending you the answers.” It’s in the present tense, so it’s the right choice when you’re attaching or including the answers right now in the message.
“Aquí le envié las respuestas” means “Here I sent you the answers.” It’s in the past tense, so it’s used when you already sent the answers before and you’re just pointing it out.
If you’re sending the answers in the same email/message, you should use “Aquí le envío las respuestas.” If you’re referring to an action already completed earlier, then “Aquí le envié las respuestas.” is correct.