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Event ID 9548. Disabled user /O=Organization/OU=SiteName/CN=RECIPIENTS/CN=User_Name does not have a master account SID (msExchMasterAccountSid). Please use Active Directory MMC to set an active account as this user's master account.
Description: Disabled user /O=Organization/OU=SiteName/CN=RECIPIENTS/CN=User_Name does not have a master account SID. Please use Active Directory MMC to set an active account as this user's master account. For more information, click .
This event is seen for instance where a user account associated with mail box is disabled and does not have an msExchMasterAccountSid as associated external account attribute is not set or vice-versa where in the disabled account is enabled which already has msExchMasterAccountSid.
Since msExchMasterAccountSid attribute does not exist on disabled user accounts, mail delivery will fail and to resolve this you need to set this attribute on the disabled user account. To resolve this, grant the SELF account Associated External Account and Full Mailbox Access permissions to the mailbox using Active Directory Users & Computers. For complete steps refer to KB article
If you have more number of disabled user accounts who does not have msExchMasterAccountSid, you have to obtain the 'NoMAS' Tool from Microsoft Product Support Services, PSS.
More information on using NoMAS is explained by Neil Hobson in his excellent article on