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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
  1. Open a Command Prompt window.

  2. Type nslookup

  3. Type set q=mx

    This sets a filter to collect only MX records and related information.

  4. 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.

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