How?
Dont know, as it depends on your mail app, and maybe version.
App help might be a starting point

It depends on your ISP (assuming that they are the ones who are handling
your mail server) and your email client.

For instance, on my email server I can configure some of the Spam
Assassin options by black/whitelisting. So I could blacklist *@cp.net.
I can set my options for spam handling so all spam meeting certain
levels is automatically deleted at the server so I never see it.

Then in my email client (Thunderbird), I can use the Junk filter and
also create my own message filters.

1. Check at your mail server (ISP if that is the way you do it) and see
if they have spam filtering and black/whitelisting.

2. Look at using message filters in your email client. You posted using
Outlook Express. If this is the email client you regularly use and you
don't know how to use message rules, look in the OE Help file or post
in an OE newsgroup. Here is one:
microsoft.public.outlookexpress.general
http://aumha.org/nntp.htm - list of MS newsgroups

Also spend some time at MVP Tom Koch's canonical OE website:
http://www.insideoe.com

3. If OE won't do what you need, try a different email client.

Malke

Assuming Outlook Express, that's what your posting with.

To block messages from a domain

In Outlook Express, click Tools | Message Rules | Blocked Senders List |
Click the Add button | Type:   cp.net    in the Address box | Click OK

To block messages from a sender or domain
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/r
ules_mailnews_block.mspx