1. Microsoft Exchange MTA Stacks service. restarting it.
2. adding the ISP’s DNS server address to the list of external DNS servers.
3.To determine whether or not a DNS configuration issue is your problem:
Open Exchange System Manager and navigate to Administrative Groups -> your administrative group -> Servers -> your server -> Protocols -> SMTP -> Default SMTP Virtual Server (or the SMTP virtual server that you are having problems with).
Right-click on the Default SMTP Virtual Server and select Properties.Select the Delivery tab and then click the Advanced button.Make sure that the server’s fully qualified domain name is entered into the Fully Qualified Domain Name field. You can verify that the name entered is valid by clicking the ‘Check DNS’ button.Now, click the Configure button and you will see a list of the external DNS servers used by the SMTP virtual server.
4. The easiest way to fix the problem is to install a faster drive (preferably a disk array) and move the queues to it. You can control the queue locations through the following registry key:
The key name is Working Directory. (You should perform a full system backup before making any registry modification.)