Confirming that the SMTP server can query DNS
When the SMTP server has an e-mail message to deliver, it must resolve the mail exchanger (MX) record and the corresponding A record of the recipient's domain. This resolution is done by means of DNS queries.
First, you need to confirm that the SMTP server can perform DNS queries. If the SMTP server cannot perform DNS queries, it will not send Internet e-mail messages. These messages will accumulate in the SMTP server's queue, and eventually delivery will fail.
Use the following procedure to confirm that the SMTP server can perform DNS queries.
To query an MX record for a domain from a command prompt
Open a Command Prompt window.
Type nslookup
Type set q=mx
This sets a filter to collect only MX records and related information.
Type domain_name**.com** where domain_name is the domain that you want to obtain the DNS records for (for example, microsoft.com or msn.com). An output similar to the following is displayed:
Server: [157.178.72.30]
Address: 157.178.72.30
microsoft.com MX preference = 10, mail exchanger = mail1.microsoft.com
microsoft.com MX preference = 10, mail exchanger = mail2.microsoft.com
microsoft.com MX preference = 10, mail exchanger = mail3.microsoft.com
microsoft.com MX preference = 10, mail exchanger = mail4.microsoft.com
microsoft.com MX preference = 10, mail exchanger = mail5.microsoft.com
mail1.microsoft.com internet address = 131.107.3.125
mail2.microsoft.com internet address = 131.107.3.124
mail3.microsoft.com internet address = 131.107.3.123
mail4.microsoft.com internet address = 131.107.3.122
mail5.microsoft.com internet address = 131.107.3.121
If the SMTP server cannot query DNS, check the server's TCP/IP settings. If the server is configured to use a public DNS server, check that you have an access rule allowing DNS traffic to the Internet from the SMTP server.