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Mailbox migration failed

By October 4, 2016September 11th, 2020Best Practices, Blog, Exchange, Office 365, Office 365
Office 365 Migrate words


When migrating users to Office 365 I’ve encountered two errors with the same fix. I wanted to share that fix with all of you.

NOTE: This is not just for O365 migrations, this could also apply to on-prem mailbox moves. The value below has changed over the years with Server OS and patch levels. Screen shots were taken from Server 2003 however this can apply to all server 2000 and newer versions.

Error 1

Transient error SourceMailboxAlreadyBeingMovedTransientException has occurred. The system will retry (2/620).

Error 2

Error details: Couldn’t switch the mailbox into Sync Source mode.
This could be because of one of the following reasons:
Another administrator is currently moving the mailbox.
The mailbox is locked.
The Microsoft Exchange Mailbox Replication service (MRS) doesn’t have the correct
permissions. Network errors are preventing MRS from cleanly closing its session
with the Mailbox server. If this is the case, MRS may continue to encounter this
error for up to 2 hours – this duration is controlled by the TCP KeepAlive settings
on the Mailbox server. Wait for the mailbox to be released before attempting to move
this mailbox again.

Typically when you see “The mailbox is locked.” you’ll find that the mailbox is locked by the Exchange backup software solution of choice. That was not the case with these two cases so it led me to more digging. I’ll spare you with all the links and suggestions that did not work.

Here’s the fix

On the source mailbox server open regedit.exe and navigate to the following reg keys

HKEY_Local_Machine -> System -> CurrentControlSet -> Services -> Tcpip -> Parameters

Create or edit your DWORD “KeepAliveTime” to 7200000 (2 hours) or if 2 hours is too rich set to 3600000 (1 hour) or 900000 (15 min)

Next, preform these same steps on your Hybrid server or that next hop Exchange server before it hits Office 365. 

Machine generated alternative text: Registry Editor File Edit View Favorites Help sppuinotiFy srvnet ssDPSRV Parameters SstpSvc stexstor storFIt storvsc storvsp swenum Linkage Parameters Performance Serviceprovider TCPIP6 TCPIP6TUNNEL tcpipreg TCPIPTUNNEL TDPIPE Name (Default) DataBasePath DeadGWDetectDeFauIt Domain DontAddDeFauItGatewayDeFauIt EnableICMPRedirect EnableIPAutoConFigurationLimits Forwardaroadcasts Hostname ICSDomain KeepAIiveTime NameServer NV Domain NV Hostname Reservedports SearchList SyncDomainWithMembership TcpTimedWaitDeIay UseDomainNameDevoIution REG REG REG REG REG REG REG REG REG REG REG REG REG REG REG REG REG REG REG sz EXPAND DWORD sz DWORD DWORD DWORD DWORD sz sz DWORD sz sz sz sz (value not set) oxoooooool (1) cablelabs com oxoooooooo (o) oxoooooool (1) oxoooooool (1) oxoooooooo (o) EXCHANGE oxoosdddoo (7200000) cablelabs com EXCHANGE 6001-6002 6004-6004 oxoooooool (1) oxoooooosc (60) oxoooooool (1) MULTI SZ sz DWORD DWORD DWORD mputer HKEY LOCAL MACHINE SYSTEM CurrentContro Set services Tcpip Parameters

 NOTE: Don’t forget to remove these changes when your migrations have finished.


Brandon Stuart, PEI



  • Andy Webb says:


    On Windows 2008 and 2012, the default is 7,200,000.

    There are several articles that suggest reducing the number significantly for security and other application issues, but can you describe the situational setup that led to this solution?

    What did the failure look like in the migration logs?


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