Quick Tip: Using a custom domain to connect to Office 365 Webmail

Office365-sign-inAfter migrating to Office 365 it can be hard to communicate the new webmail addresses to all end-users. Especially when performing a cut-over migration all users suddenly need to be informed about the new address to use.

Office 365 uses many different host names, for example portal.microsoftonline.com and login.microsoftonline.com, both taking you to the portal page where users can access all services or download desktop software. But you still have to use the Outlook link in the portal page to access your mail. How can we make this simpler for the end-users?

If your tenant is set up with ADFS you want to use the address outlook.com/yourdomain.com. This automatically logs you on directly to the webmail using your domain credentials, allowing single sign-on to Office 365. Unfortunately with a custom domain this can only be achieved using a local web server that handles the redirect to the correct web page.

So, what to do then? I don’t want to set up a web server just to handle a redirect. Luckily there is an easy workaround to this: Use DNS to create a CNAME (for example mail.mydomain.com) that point to mail.office365.com. This presents a login screen for the users, and they will then be logged on directly to the webmail page.

The CNAME can be created using PowerShell cmdlets for DNS, which was introduced in Windows Server 2012:

Add-DnsServerResourceRecordCName -HostNameAlias mail.office365.com -Name mail -ZoneName mydomain.com

If you are using ADFS the users must check the “Keep me signed in” checkbox to handle single sign-on in the future.

/ Andreas

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