[otrs] FQDN - set in SysConfig, but showing in answer message as localhost

Hi, I'm trying to set up OTRS 2.3 and most of the configuration I've got working. But though I've set the FQDN on the SysConfig page to the appropriate value, it continues to show up as localhost in the 'Compose answer for ticket' page to respond to tickets, giving '* invalid otrs@localhost (Invalid syntax)!' Including the field in the signature gives me the right value in the text, so I know the configuration is being stored, but clearly it is not being used by the compose answer page. Can anyone help me with this? thanks