OWA and Moving Mailboxes to Office 365

Lets imagine a scenario where you are using an on-premises Exchange Server and users’ use Outlook Web App, and then you move some mailboxes to the Office 365 cloud with Hybrid Coexistence enabled. The user might not know their mailbox has been moved and so yesterday they went to https://mail.company.com/owa, but today they need to visit https://outlook.com/owa/company.com (where company.com is the domain name in your login name).
But becuase the user does not know that their mailbox has been moved when they visit https://mail.company.com/owa they get an error that their OWA URL is out of date.
To fix this, and provide the user with the correct URL (https://outlook.com/owa/company.com) then you need to set the TargetOwaURL property of the Organization Relationship that you have configured for your Office 365 service domain.

Set-OrganizationRelationship name -TargetOwaURL https://outlook.com/owa/company.com

Now when users login with an account that has been moved to the cloud they will be told that their mailbox has moved, and that they should visit https://outlook.com/owa/company.com.
Some organizations though have an issue with this URL – it does not mention the company name in the domain name bit, and a name such as http://webmail.company.com/owa would be preferred for mailboxes moved to the cloud. To present the user with this URL after they login to on-premises OWA or for a URL that you can just tell them to use you need to do two things:

1. Create a CNAME record in DNS for webmail that has outlook.com as the target FQDN. The CNAME record can be anything that is not already in use for the domain (for example it could be mail if that is not in use).
2. The TargetOwaURL property of the Organization Relationship needs to be http://webmail.company.com/owa. The TargetOwaURL must finish with /owa or the on-premises OWA redirect page will error and the domain name used must be the domain name in your login name.

The outlook.com server will take the CNAME value provided by the browser and do realm discovery on this name – that is it will redirect you to the correct login server for your domain.