Imported contacts appear in red text as if an error has occurred - but none has.

When I import contacts via the drupal contact list importer module the appear in the email list field in red text surrounded by a red rectangle, which sort of looks like an error - although no error is reported either on the screen or via watchdog logs.

Mail-outs to imported lists complete as successfully. I've checked the CSS via firebug and it's coming from system.css

How can I stop this from happening?