Reader 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.