Log onto your BTMail account using a browser, not an email client/app, and then click on your username at the right hand side. Select "Settings" then go to the left hand side and select "Mail". Scroll down and click on "Blocked Senders".
Scroll down the list to view the blocked senders.
If it is a spammers domain you are more or less wasting your time trying to block the domain that is showing in the email address.
Spammers spoof their email addresses and domains so as to avoid the spam filters.
See link re spoofing.
https://www.howtogeek.com/829504/what-is-email-spoofing/
You need to look for the spammers true domain by looking at the headers of the email. If you select the email then go to the "More" button in the menu bar then click on "View Source" it will show the full details regarding the email and you should be able to find the originating domain. It is that that you need to block.
