In previous versions of OS X, you used to be able to quickly search for messages from a given domain just by typing it in on the search field. On Lion, this is a pain, because Mail tries to guess the actual sender a never lets you look at all the messages.
As a workaround, if you type "from:@domain" you will see messages from all email addresses from that domain. You can also use "to:@domain" to look for the messages that were sent to an email address at a given domain.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120313094422424