[Exim4U] group character minus

Hello I use your exim4u 2.01 on ubuntu 12.04 lts. Everything works except the email to a group that the name contains the character "-" my-office(a)mydomain.info The server responds "no such person at this address". If I send a mail to an address name-surname(a)mydomain.info work. My workaround is to create the address my-office(a)mydomain.info and enable forwarding to multiple addresses. Where is the problem with the "-" character in the groups. Thank you