Adding @DOMAIN_NAME in response to outside email

Hello. I have installed a new server. This contians a new domain (@FEFE). Inbound internet mails are received in other server (Version 5.11) wich is in other domain (@FORCEM). When I receive an internet mail and I push response in the response address appears ej: john.dumas@red.es@FORCEM. Why appears @FORCEM?Thanks in advance