Event ID - 1091

Event Id1091
DescriptionThe directory replication agent (DRA) attempted to open mail with name /o=Organization/ou=SITE/cn=Configuration/cn=Servers/cn=ServerName/cn= Microsoft DSA:ServerName but received error 2147942487. Messages could not be sent. Make sure that the mail service on this Microsoft Exchange Server computer is running.
Event InformationAccording to Microsoft:
This problem is caused because of the change in the way the directory replication communicates with the message transfer agent (MTA) in Exchange Server 4.0 and with the Information Store in Exchange Server 5.0 and Exchange Server 5.5. Directory replication backfill causes the Exchange Server computer to function as an Exchange Server 5.5 server computer, whereas the Exchange Server 4.0 executable files do not support this behavior.
To work around this problem, providing an Exchange Server 5.5 server computer exists in the site, it is possible to have this server participate in directory replication again by using the Authoritative Restore utility (authrest.exe).
This utility should be used on a directory that was backed up prior to the upgrade. It is used after restoring the directory to that server before starting the services. For clarification, see the example scenario in the More Information section below.
If an Exchange Server 5.5 server computer does not exist in the site, replication will work. However, you will not be able to create users as the updated schema will expect Exchange Server 5.5 proxy DLLs, so it will be necessary to either upgrade again immediately after fixing the original problem, or install a new Exchange Server 5.5 server computer into the site. Alternatively, editing the raw-mode values for proxy generators should correct this problem.
Reference LinksDirectory Replication Fails with 1091 Error 2147746077

Intersite Directory Replication Stops with Event ID 1091 and Error 2147942414

Backout of Upgrade Causes Replication to Fail

Replication or Knowledge Consistency Check Fails with Application Event ID 1091

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