I have set up an Exchange 2010SP3 hybrid server in addition to our existing front end / backend Exchange 2003 servers. There are only 30 mailboxes on the 2003 backend server. The plan is to move all of the mailboxes to Office 365, then retire the 2003 servers. My question is how best to move the mailboxes. Most of them are in the 1 - 10 GB range. A few of them are over 20 GB. Moving a 1 GB test mailbox took about an hour. What is the best way to move the large mailboxes with a minimum of disruption to the end users?