Block Emails By Domains In Office365
Recently I needed to block hundreds of emails in Office365 originating from domains ending in .top and a few calls to Microsoft Support were required before the rule only blocked domains ending in .top rather than also blocking domains like .topdog or subdomains like mail.topmarks.co.uk

Add this rule in your Office365 portal by selecting Admin Centers - Exchange - mail flow - rules and select The sender ... address matches any of these words and enter the domain you wish to block followed by a dollar symbol to force it to the end (.domain$)