Cause: This error is caused by duplicate System Mailbox entries in Active directory.
Complete Event Error Message:
Event ID: 7043
Task Category: IS/AD Interactions Level
Description: The mailbox GUID of an external system mailbox (‘Mailbox – SystemMailbox{5795ed94-6402-4198-a675-522cd342b1a2}’) does not match the information in the Active Directory for the mailbox. The existing GUID (‘7403d94d-1a74-4a9b-b6b0-34db1b921328: /o=ABC/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=SystemMailbox{5795ed94-6402-4198-a675-522cd342b1a2}’) has been replaced with the expected GUID (‘da373d09-c4d9-4e4e-b7b0-cb4f0ae818f0: /o=ABC/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=SystemMailbox{5795ed94-6402-4198-a675-522cd342b1a2}’).
Investigation: In order to review the accounts, open Active Directory Users & Computers, click the View menu at the top and check “Advanced Features” and expand the “Microsoft Exchange System Objects” container.
Look for an entry with “CNF” in the middle which is duplicate to another entry:
SystemMailbox{5795ed94-6402-4198-a675-522cd342b1a2}CNF:2a2a5e4e-a075-4e7c-af93-a955971b1255
SystemMailbox{5795ed94-6402-4198-a675-522cd342b1a2}
Resolution:
1) Backup Active Directory
2) Delete the duplicate system mailbox that has the CNF in between the GUID:
SystemMailbox{5795ed94-6402-4198-a675-522cd342b1a2}CNF:2a2a5e4e-a075-4e7c-af93-a955971b1255