Updating user and group for exim4

With the current user and group we are getting the next error:
"chown: invalid user: `exim:mail'"

Tested in Ubuntu 12.04.
Exim default name details here: https://wiki.debian.org/PkgExim4UserFAQ#The_user_name_Debian-.24PACKAGE_is_too_long:_it_misaligns_ls_output_and_is_truncated_by_ps_and_atop._And_it.27s_ugly.21