I'd try something like:
^(?:.+\@(?:domain1.com|domain2.com|domain3.com)|\s*)$
Jason Rhinelander
Gossamer Threads
jason@gossamer-threads.com
^(?:.+\@(?:domain1.com|domain2.com|domain3.com)|\s*)$
Jason Rhinelander
Gossamer Threads
jason@gossamer-threads.com