PQ69935: SMTP - 550 HOST 'XXXXX.COM' UNKNOWN - MESSAGE WHEN HOST EXISTS

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• Mail is being returned to a user with message:
  xxxx.xxx.xxx unable to deliver note to <postmaster@yyyyy.com>
  .
  550 Host 'yyyyy.com' Unknown
  A resolver trace in SMTP shows that SMTP asks for all "MX" and
  "A" records for yyyyy.com. The nameserver replies back with
  a number of "MX" records and "A" records.
  SMTP then goes out to the nameserver and tries to get the
  missing "A" record but gets back:
  #5524 Remote Nameserver down/unable to perform recursion
  #5487 Popping request from Wait to SendQ
  After the third try to get the "A" record the mail is returned
  with HOST UNKNOWN message.
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of the IBM Communications Server   *
  *                 for OS/390 Release 10 IP, z/OS Version 1     *
  *                 Release 2 IP and Release 4 IP: SMTP          *
  ****************************************************************
  * PROBLEM DESCRIPTION: Mail is being incorrectly returned to a *
  *                      user with the "550 Host Unknown"        *
  *                      message.                                *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  A mail domain has multiple hosts defined to handle its
  mail. If the last host in the MX list is not defined in
  the name domain space, a piece of mail destined for that
  mail domain will not be delivered.  The mail will be
  returned to the sender as undeliverable.
  This problem will occur only when the last name server's
  response for the undefined host is not authoritative or when
  the query times out.
  +-------------------------------------------------------------+
  + Please check our Communications Server for OS/390 homepages +
  + for common networking tips and fixes.  The URL for these    +
  + homepages can be found in Informational APAR II11334.       +
  +-------------------------------------------------------------+
  

Problem conclusion

• In the MX host list processing in procedure SendRQR (SMTPRES),
  a check was added to see if the unresolved MX host is the last
  MX host.  If this is the last host and we have valid addresses,
  then the host is ignored by deleting its name from the list of
  viable MX hosts.  Since this is the last MX host, resolution is
  done.
  
  * Cross Reference between External and Internal Names
  EZABB025 (SMTPRES )  EZABB025 (SMTPRES )  EZABB025 (SMTPRES )
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

  UQ73919 UQ73920 UQ73921

Modules/Macros

• EZABB025
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R120 PSY UQ73919

     UP03/03/04 P F302

• R140 PSY UQ73920

     UP03/03/04 P F302

• R50A PSY UQ73921

     UP03/03/04 P F302

Fix is available

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