Formal:
* Mensaje no enviado: This is the most literal and direct translation.
* El mensaje no se ha enviado: This is slightly more formal and emphasizes the action of not sending.
Informal:
* No se envió el mensaje: This is a more casual way to say it.
* El mensaje no salió: This is a more colloquial expression, similar to "the message didn't go through."
Specific scenarios:
* "El mensaje no se pudo enviar" (The message could not be sent) - This is useful if the problem is with the recipient, network, or some other technical issue.
* "El mensaje no se ha entregado" (The message has not been delivered) - This is used when the message was sent but hasn't reached the recipient yet.
The best translation for you will depend on the specific situation and your desired level of formality.