Exchange. Queues stops. 451 4.4.0 Primary query Failed

Problem: 

The Exchange queues grow. The emails can't be send

Symptom:

The emails can't be send. The email queues grow.

On the Exchange console we have the message error "451 4.4.0 Primary query Failed”

Solution:

Go to "external DNS lookups" and change the parameter.

If you return to this menu to check it, the option will be  "(Todas las direcciones IPv4 disponibles)"

You could disable and enable someone queue to check.

Gon. October 2016