When ADFS is deployed in conjunction with Office 365 it prevents users from having to re-enter their corporate passwords in order to access web-based Office 365 services. However, the user is still required to input their email address. When the email address is entered, the Office portal redirected the users to the ADFS URL which was configured as part of the federation between ADFS and Office 365. In an ideal world it would be possible to auto populate this email address field to increase the user experience further.
One of the easiest ways to achieve this with no further configuration to ADFS or the users browser is to use a customized URL to access Office 365 services. This basically appends the customers domain to the URL.
It is possible to code this behavior into the ADFS code the following blog post includes a snippet of C# code that sets this. None of the content in this blog post is produced or owned by me.
For this environment, there are only 25 named users therefore the customized URL is good enough. However it is good to know it is possible to set this statically if required.