MAPI errors when moving mailboxes

I spent the better part of the day today working through a problem with the Move Mailbox wizard. I was in the final phase of an Exchange 5.5 to Exchange 2003. Moving the mailboxes one or two at a time seemed to work okay, but they would timeout when more than two or three mailboxes were being moved at a time. The message in the move mailbox log showed:

errorCode='0xc1050115'
An unexpected, unknown error has occurred.MAPI or an unspecified service provider.
ID no: 80040115-0000-00000000

The following event showed up in the event viewer each time a move mailbox failed:

Event Type: Error
Event Source: MSExchangeAdmin
Event Category: Move Mailbox
Event ID: 1023
Date: 1/19/2006
Time: 9:26:13 AM
User: N/A
Computer: xxxxxxxx
Description:Unable to set a property on the message store on 'xxxxxx'. Result: Network problems are preventing connection to the Microsoft Exchange Server computer.An unexpected, unknown error has occurred.Microsoft Exchange Server Information StoreID no: 80040115-0514-000006be

My first instinct was network problems. We hard-coded the network adapters on both the source and destination servers to 100Mb/s, full duplex and put the servers on a different switch. None of these things had any affect, so I did Google searches and came up with possibilities such as corrupted messages. After running ESEUTIL and ISINTEG a couple of times with no success, we were just about to use ExMerge to archive everything to PST and then move the mailboxes once they were smaller.

However, simply copying the first mailbox's PST file (80MB) took 30 minutes. However, it copied fine from the 5.5 server to a different server. So, yep, you guessed it, something about the network adapter was not working properly. We switched it over to a GB Ethernet switch and everything was just shiny!

I never got to the bottom of why network performance for that one server was so bad when it was in a 100MB switch, but the moral of the story is to not forget the basics.