Can I block email from certain sites?

Since you mentioned you're running Outlook, I'll start there. Using Outlook 2007 as our example, click on Tools, Options, and in the Preferences tab, click on Junk Email.... In the resulting "Junk E-mail Options" dialog, click on the Blocked Senders tab, and then the Add... button:

You can see I've entered "@somerandomservice.com". Press OK, and the list of blocked entries looks like this:

Now any email that arrives which is from any address @somerandomservice.com will be treated as junk. You can enter specific email addresses in this list, and/or domains, as I've done, which will block all email from that domain.

Now, it's important to realize that this occurs in your email program - Outlook - after the email has been downloaded. Chances are Outlook will simply move that incoming message directly to the junk mail folder once it's been downloaded if the sender is on the block list.

Other email programs may not support blocking users or domains directly, but you can simulate this using rules or filters. For example, in Thunderbird:

This filter causes all email from somerandomservice.com to be marked as Junk. Naturally you can create as many filters as you need.

In Outlook Express, this should work:

In all of the cases above, the email is downloaded, and then acted upon by your email program's filters. In order to prevent the email from even being downloaded you'll need to check with your email service provider - some provide server-side block lists.

Web based email services often also provide block lists. For example, in Windows Live Hotmail, you can click on the Options menu, More Options, and then under Junk e-mail click on Safe and blocked senders, and finally click on Blocked senders:

GMail apparently doesn't have a block feature, but much as we did above, we can set up a filter to automatically delete email from a domain:

As you can see the tools available and the techniques you need to use vary greatly. You might be able to explicitly block a sender or a sender's domain, either in your email program, or on your server. If not, you might be able to use your email program or service's filtering capabilities to effectively block email from a specific domain or sender by simply deleting it.

But you'll need to investigate the options that you have and act accordingly.