This does indeed seem like a bug with this version of Unity. I found similar fault related to version 5, details below, I hope this helps:
CSCsj43233 Bug Details
Ex2000/2003: MAPI address book providers Global Catalog failover broke
Symptom:
1. Afer addressing a message to another subscriber and sending the message via the telephone user interface (TUI), you do not hear "message sent". Instead you are returned to the main mailbox menu (i.e. "For new messages press 1, to send a message press 2" etc.), and the recipient does not recieve the message.
Conditions:
1. This applies to Cisco Unity 5.0(1) only, this defect does not apply to earlier versions of Cisco Unity.
2. This defect applies to Cisco Unity when when the partner mail server is Exchange 2000 or 2003 only, this does not apply to Cisco Unity when partnered with Exchange 2007.
3. This condition is created when their is more than one available Global Catalog Server and the Global Catalog server that the Cisco Unity servers MAPI address book provider is using to resolve recipient addresses is shutdown or rebooted.
Workaround:
In the Windows registry, under HKLM\Software\Active Voice\MALEx\1.0\
Change the value specified for "NegAuth" from 1 to 0 and restart Cisco Unity.
NOTE: If Exchange 2007 is the partner mail server this setting must remain set to 1 or you will create a problem with similiar symptoms.
Further Problem Description:
The following events are logged in the Windows Application Event Log:
Event Type: Warning
Event Source: CiscoUnity_MALEx
Event Category: Warning
Event ID: 30019
Date: 6/28/2007
Time: 7:20:00 PM
User: N/A
Computer:
Description:
The MAPI subsystem has indicated that the Global Catalog Server
. which is used to resolve addresses for message submission cannot be reached, and that it has switched to using Global Catalog server .. Unity will continue to function using this newly selected Global Catalog server and will not automatically switch back to the original one. If Unity does not have a dedicated connection with sufficient bandwidth to the newly selected server, then there may be significant delays in Exchange access by Unity. Please verify that Unity has a good connection to the new Global Catalog for proper functioning.
Event Type: Error
Event Source: CiscoUnity_ConvMsg
Event Category: Network
Event ID: 10015
Date: 6/28/2007
Time: 7:20:00 PM
User: N/A
Computer:
Description:
IAvDohMessage::Submit: A recipient possibly lacked an email box: Recipients=([SubscriberA] ) returned [0x80040920] on line 976 of file E:\Views\CU5.0.0.297\un_Core2\ConversationEng\AvStateSvr\AvSMsgSend.cpp
Event Type: Error
Event Source: CiscoUnity_ConvMsg
Event Category: Network
Event ID: 10012
Date: 6/28/2007
Time: 7:20:00 PM
User: N/A
Computer:
Description:
This message has the following recipients : ([SubscriberA] ). The recipient mailboxes may be disabled or inaccessible, Cisco Unity may be having trouble connecting with the partner server, or the Cisco Unity messaging component may not be properly configured. 979
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