I have received the follow question a couple of times, so I thought I would write a blog post about it.
If it is possible to add a more intuitive and simple URL for mail hosted on Office 365?
It is possible and described below.
The known URL for Office 365 is: http://portal.microsoft.com – from there the users can access Outlook and SharePoint. But what if end users, just want to quickly access Exchange OWA.
And the portal.microsoft.com URL might not even be the easiest for many end users to remember, when they are used to something like mail.contoso.com.
The direct URL for Exchange Online (Office 365) is: mail.office365.com
It is possible to update your DNS records to point at this DNS record.
Create a CNAME for mail.contoso.com which points to mail.office365.com
The above is the easy way of supplying your end users, with a new URL for OWA (Office 365).
Things are a lot different in Hybrid scenarios, see the article below for reference to hybrid scenarios: http://community.office365.com/en-us/w/officeapps/simplify-the-owa-url-for-office-365-hybrid.aspx