You are moving Exchange mailboxes of users from one datacentre to another. Post movement, your users will be able to access their mailboxes via OWA URL of the new datacentre. However, until reconfigured, their Outlook application will still be connecting to user mailbox via their previous datacentre OWA URL only and thus email access via Outlook becomes flaky and slower. If the previous datacentre OWA URL becomes unavailable, users will not be able to access their mailboxes until their OWA URL configured in Outlook is updated to point to the new datacentre OWA URL.
Update user Outlook Account/Profile Settings with new datacentre OWA URL as detailed below:
Start > Outlook > Account Settings > E-mail > double click on the configured URL account > On Server Settings
Click More Settings
on Microsoft Exchange > click “Exchange Proxy Settings” > in Connection Settings
update the URL with OWA URL host name.