To fix this problem you need to remove or comment out "unseen" from your exim.conf.
Example:
virtual_aliases_nostar:
driver = redirect
allow_defer
allow_fail
data = ${if exists{/etc/virtual/$/aliases}{${lookup{$local_part}lsearch{/etc/virtual/$/aliases}}}}
file_transport = address_file
group = mail
pipe_transport = virtual_address_pipe
retry_use_local_part
#unseen
#include_domain = true
Reference: www.directadmin.com/forum/showthread.php?threadid=976