Outlook web app in Office 365 also allows users to block a sender or a domain name. This is similar to block sender feature available as part of Outlook 2010.
In the Office 365 Home page, navigate to Outlook section then click the Options link.
In the Outlook Web App page, click the Block or Allow link available under the Account Section.
In Block or Allow screen, navigate to Blocked Senders section.
Now enter the email address or the domain name in Enter a sender or domain here and press + icon available at end of the text box.
For example if you want exclude emails form tz.com domain name, then enter domain name in the text box and click the Add icon. The entered list will now be added to the Blocked Senders list in Outlook Web App.