In versions of OS X before Lion, you could quickly search for messages from a given domain (macworld.com, for example) just by typing it in the search field. But such searches are more difficult in Lion's version of Mail. That's because, when you type the domain name, the program will show you all of the messages that have been sent to or received from that domain, as well as any messages that contain that domain as a string of text in the body; it will also show you a drop-down suggestion-list of specific senders from that domain.
Reader pfernandes points out that there is a way to refine Mail searches to find all senders or recipients from a given domain: If you type from:@domain, you will see a list of all the messages you've received from any email addresses in that domain. You can also use to:@domain to look for the messages that you've sent to any addressees in it.
Note that you will see these search results in Mail's message-list pane after you type the search-term but before you hit Return. If you then hit Return, Mail will convert your search term into a token and treat it literally; in other words, it will look for senders (or recipients) whose addresses consist solely of the domain name and therefore return no results.