I am having trouble with my migration from 2010 to 2016. I've made a lot of progress, I have the old 2010 server with all the mailboxes running and the new 2016 server handling all the ActiveSync and OWA requests via proxy. It works fine, no issues. The existing local outlook users using OL2010 and higher are connected without a problem. I migrated one mailbox as a test and that client is not able to connect, it asks for the users password every time they open outlook, but when they type it in it doesn't work, just pops right back again and again and never connects to the server. That same users mobile devices and also OWA work just fine no issues, it is on local outlook that doesn't work. I have been searching all day and have no solutions so I'm hoping someone out there has had this problem and has a solution, I have looked through the logs and am not able to find anything useful. I'm not an exchange expert but I have been managing this server for years and need help please.