How do you block someone when their email address format isn’t recognised on the blocked senders page

How do you block someone when their email address format isn't recognized on the blocked senders page?  When i type it in it says 'it must be so and so, not this' but in that case how did it come to be used as an email address in the first place?  Address is
postmaster​@bt884593po​p.onmicroso​ft.com and hotmail are unable to block it apparrently