This article was previously published under Q263237
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Most programs use User Datagram Protocol (UDP) queries to resolve host names. Only when the contents of the query are larger than the maximum packet size does the program need to use Transmission Control Protocol (TCP).
The Simple Mail Transfer Protocol (SMTP) service is unique, because the SMTP service uses TCP queries by default.
If the Domain Name System (DNS) server does not support TCP queries, the query attempt is reset. Lookup for mail delivery does not work and the following event is logged:
Event ID 4000 Source: SMTPSVC Type: Warning Description: Message delivery to the remote domain "your_domain.com" failed. An internal DNS error caused a failure to find the remote server. 0000: c00402e7
Request for Comments (RFC) 883 and the later RFCs prescribe that DNS servers be able to accept both TCP virtual circuits and UDP.
A Network Monitor trace may show a TCP connection attempt to the destination Internet Protocol (IP) of the DNS server on port 53. If the connection is reset, the DNS server may not support TCP queries and you may need to use a different DNS server.
To test whether TCP queries work, perform the following steps on the SMTP server:
From a command prompt, type nslookup, and then press ENTER. You should see the following output: