outlook emails spamming my server

Hi I am receiving literally thousands of fake Outlook accounts onto my server that are actually stopping my "genuine" emails as my server is bogged down with this attack. Here are a sample of the emails :- *** Email address is removed for privacy *** *** Email address is removed for privacy *** *** Email address is removed for privacy *** *** Email address is removed for privacy *** *** Email address is removed for privacy *** *** Email address is removed for privacy *** literally thousands of junk email coming from outlook. Any ideas on how to get rid of this ****? Its obviously a cyber attack on my system but its nearly all coming from Outlook emails.