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XFOR: Internet Mail Service Not Trying Next Exchange Record

The information in this article applies to:

Microsoft Exchange Server, versions 5.0, 5.5

SYMPTOMS

The Microsoft Exchange Server Internet Mail Service does not fail over to the next Exchange record for a given domain, and the message is eventually returned to the sender with a non-delivery report (NDR).

CAUSE

The Internet Mail Service failed to try the next Exchange record if the host for the first Exchange record accepted the TCP three-way handshake (Syn-SynAck-Ack).

STATUS

Microsoft has confirmed this to be a problem in Exchange Server version 5.0.

A supported fix is now available, but has not been fully regression-tested and should be applied only to systems experiencing this specific problem. Unless you are severely impacted by this specific problem, Microsoft recommends that you wait for the next Service Pack that contains this fix. Contact Microsoft Technical Support for more information.

This fix has been posted to the following Internet location:

ftp://ftp.microsoft.com/bussys/exchange/exchange-public/fixes/Eng/Exchg5.0/Post-SP2-IMS/

Microsoft has confirmed this to be a problem in Microsoft Exchange Server version 5.5. This problem has been corrected in the latest U.S. Service Pack for Microsoft Exchange Server version 5.5. For information about obtaining the Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

S E R V P A C K

MORE INFORMATION

By examining a network trace of a computer exhibiting this behavior, you will see the following conversation occur:

The Internet Mail Service queries the Domain Name Service (DNS) server for the domain in question.

For example, in the following packet, a query has been generated to find all Exchange records for SampleOrg.com, as can be seen in the DNS: Question Section below:

    + FRAME: Base frame properties
    + ETHERNET: ETYPE = 0x0800 : Protocol = IP:  DOD Internet Protocol
    + IP: ID = 0x6F0A; Proto = UDP; Len: 57
    + UDP: Src Port: Unknown, (1146); Dst Port: DNS (53); Length = 37 (0x25)
     DNS: 0x5:Std Qry for SampleOrg.com. of type Mail Xchg on class INET addr.
         DNS: Query Identifier = 5 (0x5)
       + DNS: DNS Flags = Query, OpCode - Std Qry, RD Bits Set, RCode - No
    error
         DNS: Question Entry Count = 1 (0x1)
         DNS: Answer Entry Count = 0 (0x0)
         DNS: Name Server Count = 0 (0x0)
         DNS: Additional Records Count = 0 (0x0)
         DNS: Question Section: SampleOrg.com. of type Mail Xchg on class INET
    addr.
             DNS: Question Name: SampleOrg.com.
             DNS: Question Type = Mail exchange
             DNS: Question Class = Internet address class

The DNS server returns the list of Exchange records it has for the domain in question.

For example, in the packet, the DNS server responds as can be seen in the DNS: Answer Section below. From this section of the packet, we know that the DNS server knows of two Exchange records for SampleOrg.com: Server1 with a preference of 100 and Server2 with a preference of 300.

    + FRAME: Base frame properties
    + ETHERNET: ETYPE = 0x0800 : Protocol = IP:  DOD Internet Protocol
    + IP: ID = 0xC482; Proto = UDP; Len: 527
    + UDP: Src Port: DNS, (53); Dst Port: Unknown (1146); Length = 507 (0x1FB)
     DNS: 0x5:Std Qry Resp. for SampleOrg.com. of type Mail Xchg on class INET
     addr.
         DNS: Query Identifier = 5 (0x5)
       + DNS: DNS Flags = Response, OpCode - Std Qry, AA RD RA Bits Set, RCode
     - No error
         DNS: Question Entry Count = 1 (0x1)
         DNS: Answer Entry Count = 2 (0x2)
         DNS: Name Server Count = 5 (0x5)
         DNS: Additional Records Count = 2 (0x2)
         DNS: Question Section: SampleOrg.com. of type Mail Xchg on class INET
     addr.
             DNS: Question Name: SampleOrg.com.
             DNS: Question Type = Mail exchange
             DNS: Question Class = Internet address class
         DNS: Answer section: SampleOrg.com. of type Mail Xchg on class INET
     addr.(8 records present)
             DNS: Resource Record: SampleOrg.com. of type Mail Xchg on class
     INET addr.
                 DNS: Resource Name: SampleOrg.com.
                 DNS: Resource Type = Mail exchange
                 DNS: Resource Class = Internet address class
                 DNS: Time To Live = 7200 (0x1C20)
                 DNS: Resource Data Length = 10 (0xA)
                 DNS: Preference = 100 (0x64)
                 DNS: Exchange Mailbox: Server1.SampleOrg.com.
             DNS: Resource Record: SampleOrg.com. of type Mail Xchg on class
      INET addr.
                 DNS: Resource Name: SampleOrg.com.
                 DNS: Resource Type = Mail exchange
                 DNS: Resource Class = Internet address class
                 DNS: Time To Live = 7200 (0x1C20)
                 DNS: Resource Data Length = 11 (0xB)
                 DNS: Preference = 300 (0x12C)
                 DNS: Exchange Mailbox: Server2.SampleOrg.com.
       + DNS: Authority Section: SampleOrg.com. of type Auth. NS on class INET
      addr.(5 records present)
       + DNS: Additional Records Section: Server1.SampleOrg.com. of type Host
      Addr on class INET addr.(2 records present)

The Internet Mail Service queries the DNS server for the Host information of the Exchange record with the lowest preference value.

For example, in the following packet, a query has been generated to find the Host Address for Server1.SampleOrg.com, as can be seen in the DNS: Question Section below.

    + FRAME: Base frame properties
    + ETHERNET: ETYPE = 0x0800 : Protocol = IP:  DOD Internet Protocol
    + IP: ID = 0x700A; Proto = UDP; Len: 64
    + UDP: Src Port: Unknown, (1147); Dst Port: DNS (53); Length = 44 (0x2C)
      DNS: 0x1:Std Qry for Server1.SampleOrg.com. of type Host Addr on class
     INET addr.
         DNS: Query Identifier = 1 (0x1)
       + DNS: DNS Flags = Query, OpCode - Std Qry, RD Bits Set, RCode - No
     error
         DNS: Question Entry Count = 1 (0x1)
         DNS: Answer Entry Count = 0 (0x0)
         DNS: Name Server Count = 0 (0x0)
         DNS: Additional Records Count = 0 (0x0)
         DNS: Question Section: Server1.SampleOrg.com. of type Host Addr on
     class INET addr.
             DNS: Question Name: Server1.SampleOrg.com.
             DNS: Question Type = Host Address
             DNS: Question Class = Internet address class

The DNS returns the IP address of the server in question.

For example, in the following packet, the DNS server responds as can be seen in the DNS: Answer Section below. From this section of the packet, you see that the IP address of Server1 is 200.200.200.1.

    + FRAME: Base frame properties
    + ETHERNET: ETYPE = 0x0800 : Protocol = IP:  DOD Internet Protocol
    + IP: ID = 0xC483; Proto = UDP; Len: 274
    + UDP: Src Port: DNS, (53); Dst Port: Unknown (1147); Length = 254 (0xFE)
      DNS: 0x1:Std Qry Resp. for Server1.SampleOrg.com. of type Host Addr on
      class INET addr.
         DNS: Query Identifier = 1 (0x1)
       + DNS: DNS Flags = Response, OpCode - Std Qry, RD RA Bits Set, RCode -
      No error
         DNS: Question Entry Count = 1 (0x1)
         DNS: Answer Entry Count = 1 (0x1)
         DNS: Name Server Count = 5 (0x5)
         DNS: Additional Records Count = 5 (0x5)
         DNS: Question Section: Server1.SampleOrg.com. of type Host Addr on
      class INET addr.
             DNS: Question Name: Server1.SampleOrg.com.
             DNS: Question Type = Host Address
             DNS: Question Class = Internet address class
         DNS: Answer section: Server1.SampleOrg.com. of type Host Addr on
      class INET addr.
             DNS: Resource Name: Server1.SampleOrg.com.
             DNS: Resource Type = Host Address
             DNS: Resource Class = Internet address class
             DNS: Time To Live = 636 (0x27C)
             DNS: Resource Data Length = 4 (0x4)
             DNS: IP address = 200.200.200.1
       + DNS: Authority Section: SampleOrg.com. of type Auth. NS on class INET
      addr.(5 records present)
       + DNS: Additional Records Section: ns.SampleOrg.com. of type Host Addr
      on class INET addr.(5 records present)

The Internet Mail Service now initiates a TCP connection to Server1.

This is done with the following three packets, which is known as the TCP three-way handshake (Syn-SynAck-Ack).

    + FRAME: Base frame properties
    + ETHERNET: ETYPE = 0x0800 : Protocol = IP:  DOD Internet Protocol
    + IP: ID = 0x730A; Proto = TCP; Len: 44
    + TCP: ....S., len:    4, seq:   3447617-3447620, ack:         0, win:
    8192, src: 1145  dst:   25 (SMTP)
   
    + FRAME: Base frame properties
    + ETHERNET: ETYPE = 0x0800 : Protocol = IP:  DOD Internet Protocol
    + IP: ID = 0x292B; Proto = TCP; Len: 44
    + TCP: .A..S., len:    4, seq:2120192000-2120192003, ack:   3447618, win:
    4096, src:   25 (SMTP)  dst: 1145
   
    + FRAME: Base frame properties
    + ETHERNET: ETYPE = 0x0800 : Protocol = IP:  DOD Internet Protocol
    + IP: ID = 0x740A; Proto = TCP; Len: 40
    + TCP: .A...., len:    0, seq:   3447618-3447618, ack:2120192001, win:
    8760, src: 1145  dst:   25 (SMTP)

IMPORTANT NOTE: The following packet is never received in the failing case. In a normal working situation (that is, there is an SMTP daemon listening on Server1), the next packet contains the SMTP Service Ready banner. If Server1 does not send an SMTP Service Ready banner, the Internet Mail Service continues to try this same Exchange record until the message times out and an NDR is sent to the sender.

    + FRAME: Base frame properties
    + ETHERNET: ETYPE = 0x0800 : Protocol = IP:  DOD Internet Protocol
    + IP: ID = 0x9485; Proto = TCP; Len: 118
    + TCP: .AP..., len:   78, seq:2034176001-2034176078, ack:   3332995, win:
    4096, src:   25 (SMTP)  dst: 1145
     SMTP: Rsp: Service ready, 78 bytes
         SMTP: Response = 220 Server1.SampleOrg.com Microsoft Exchange
    Internet Mail Service 5.0.1458.49 ready

Additional query words: IMC

Keywords          : kbusage XFOR 
Version           : 5.0 5.5
Platform          : winnt 
Issue type        : kbbug