Having done cross-forest migrations since the Exchange 5.5 days, let me say that they are complex, there are many things that can go wrong and that it is almost ALWAYS cheaper to do it without expensive tools as long as you are careful and do extensive testing.

First, let me start off with one of the first thing you will likely see in your tests:

“Failed to set basic mailbox information, will retry in 60 seconds”

This error is recoverable. In fact, it is really just a warning that it took a little longer to stage the target Exchange Mailbox.

So what is happening is Move-Mailbox is in the process of Mailbox-enabling the target AD account and it was unable to confirm the creation. The tool is designed to wait a while and then confirm its existence again. Now, here is where it gets strange….if you search enough you will find the Move-Mailbox command line reference on TechNet.

http://technet.microsoft.com/en-us/library/aa997599.aspx

This article includes the following details on Retry Interval:

RetryInterval

Optional

Microsoft.Exchange.Data.EnhancedTimeSpan

The RetryInterval parameter specifies the interval for retrieving the move’s status from the server.

Unfortunately, Microsoft leaves out two very important pieces of information:

1)      The Syntax is 00:00:00 (Hours:Minutes:Seconds)

2)      No matter what you use as a valid, the status screen will continue to show a 60 second delay ““Failed to set basic mailbox information, will retry in 60 seconds”

For Syntax and testing, I would recommend you start with 00:00:30 which is essentially half the default period. Your Move-Mailbox Script should look something like this:

Move-Mailbox -identity $item -TargetDatabase $TargetExchange -GlobalCatalog $GC -SourceForestGlobalCatalog $SGC -NTAccountOU $OU -SourceForestCredential $SourceCredential -TargetForestCredential $TargetCredential -confirm:$False -BadItemLimit 0 -RetryInterval 00:00:45 -SourceMailboxCleanupOptions MailEnableSourceAccount -ReportFile $report

Now, here is where things get fun. After you add the –RetryInterval 00:00:45 the status screen will still say “will retry for 60 seconds” but if you time it you will see that the value you entered is honored.

In the migrations I have done, I have found that 30-45 seconds seem to work best for me. Also, I have also found that while Move-Mailbox tends to do a pretty good job at moving messages, it does not seem to be as robust when working with the AD. The performance of the DC, the proximity to the script execution and other less-measurable factors can easily trip this up in a cross-forest migration. I recommend that you push as much of the AD tasks outside of Move-Mailbox in order to increase the effectiveness of your migration. In short, try to do the Account Setup and Account cleanup tasks OUTSIDE of Move-Mailbox.

I have some scripts, a project overview and other helpful suggestions for a Cross-Forest move of mailboxes. Stay tuned!