The SMTP server's RESOLVERUSAGE statement indicates if domain name resolution is to be used or not. If name resolution is not wanted, RESOLVERUSAGE NO should be specified. See Step 9: Enable sending of non-local messages to other mail servers.
If the RESOLVERUSAGE statement is not specified or is specified as RESOLVERUSAGE YES, the SMTP server will resolve domain names. Resolver TCPIP.DATA statements must be configured before you can use domain name resolution for SMTP. For a description of how TCPIP.DATA statements can be specified, see The resolver.
For more information on the SMTP RESOLVERUSAGE statement and the TCPIP.DATA resolver statements, see z/OS Communications Server: IP Configuration Reference.
To use a domain name server, configure the TCPIP.DATA data set with the IP address of one or more name servers. If the TCPIP.DATA data set does not point to any name servers, the local site tables are used by SMTP. However, if the SMTP server is configured to use name servers, SMTP does not use the site tables.
To determine which DSN the SMTP server is using, look for message number EZA5231I in the output data set specified by the OUTPUT statement in SEZAINST(SMTPPROC).
When SMTP uses a domain name server, it asks the domain name server for the MX records for the host to which it is trying to connect. If SMTP does not find MX records for a host, it delivers mail only to the primary host listed in the A records. The MX and A records are coded in the domain name server database.
The basic idea behind MX records is to send the mail as close as possible to the final destination. The destination host may currently be inactive, for example, because it is in another time zone. SMTP needs a synchronous connection to deliver the mail, but due to the different time zones, two systems might never be active at the same time and would never be able to exchange mail.
Using MX records would allow the SMTP server to deliver the mail to an alternate host if the first one is unavailable. SMTP tries to deliver mail to the host with the lowest MX record count. If the host is not currently available, it tries the host with the next lowest count.
For example, if SMTP wants to send mail to USER@BASKET, it checks the name server for MX records and finds the following records:
MVS20 BASKET A
BASKET MX 0 MVS20
BASKET MX 5 MVS18
BASKET MX 10 VMQSMTP delivers the mail to the BASKET with the lowest count on its MX record. If MVS20 is unable to receive the mail, SMTP then tries to deliver it to MVS18. If MVS18 cannot receive the mail, it tries VMQ. If none of the hosts can receive the mail, SMTP stores the mail and queues it for later delivery, at which time the process repeats.
For more information about how to add MX records to your name server, consult RFC 974, “Mail Routing and the Domain System.”
To receive a detailed trace on how SMTP is resolving a particular host name, you can issue the SMSG SMTP TRACE command at the console or use a SYSTCPT DD statement in the SMTP cataloged procedure. You can also add the TRACE RESOLVER statement when configuring the TCPIP.DATA data set, but this will also trace the name resolution for all the other applications using the name server. To prevent the console log from becoming too large, use the TRACE RESOLVER statement only for debugging.
If changes to the domain name server requires you to resolve already queued mail again, use the SMSG SMTP EXPIRE command as described in the z/OS Communications Server: IP User's Guide and Commands. You can also query operating statistics, such as mail delivery queues of the SMTP server, by using the SMSG SMTP command. This and other administrative tasks are discussed in more detail in the z/OS Communications Server: IP User's Guide and Commands.