40 Host Or Domain Name Not Found Mail New

I am an administrator of a exchange server who is seeing this same problem.
Host or domain name not found mail. Telnet ip port 25 doesn t work but telnet works for the local ones. Mail user host domain mailq or more var log mail log i either get connection timed out or no route to host or host not found try again some errors are from trying to email external servers which might be prevented because isp s block it i think. Email address is removed for privacy technical details of permanent failure.
Or do you use one of the email clients that comes with hosting. Delivery to the following recipient failed permanently. Would you be able to send a screen shot of your dns settings.
In this case it is not. I am using raspbian and the postfix version 2 9 6 service seems to be automatically run when i boot but if i want to send a mail the email stays in the queue.