When the context is to send something, a letter for example, both are correct, but "mandar" can has other meaning depending in what context are you using it, "mandar" means command too. Cheers.

Hay una diferencia, cuando "mandar" NO es igual a enviar. Mi mujer manda mucho, My wife command too much.

no difference in meaning, however, "mandar" can refer to receive an order must be obeyed, for example, mi mama me mando a la tienda a comprar huevos. The difference would be that "mandar" is used more commonly in everyday language, and "envíar" is a more formal word

Hello, Bill. No hay una diferencia significativa. Tú puedes decir "Yo envié/mandé la carta" (I sent the lettter) Dependiendo del caso, preferiría usar "enviar" para casos más formales.