With the Sender Blacklist/Whitelist, you can block specific sender addresses. Both envelope sender addresses and the "From:" or "Reply-To:" headers are compared against the list. You can use "*" as a wildcard in this list. Here are some possible patterns:
* user@domain.com (complete email addresses)
* *@*domain.com (block complete domains)
* user@* (block specific usernames for all domains)