This is easy but a lot depends on how far down you want to block. See below:

*yeah.net will block any email that has yeah.net in it.

While *.yeah.net will block emails that have .yeah.net ie someone@host.yeah.net but let emails from someone@yeah.net pass through.

Putting the * in front of a dot will only block hosts from a domain while leaving the dot out will block all emails from the domain. .

Hope this helps.

G