Probably a permissions issue.
Code:chown -R username:mail /var/spool/virtual/domain
server:~# ls -ld /var/spool/virtual
drwxrwxrwt 3 mail mail 4096 2007-05-26 07:15 /var/spool/virtual
server:~# ls -ld /var/spool/virtual/domain.tld
drwx------ 2 admin mail 4096 2007-05-28 15:59 /var/spool/virtual/domain.tld
server:~# ls -l /var/spool/virtual/domain.tld
total 744
-rw-rw---- 1 admin mail 754169 2007-05-28 15:59 sazpaimon
server:~# ls -ld /var/spool/virtual/domain.tld
drwx------ 2 admin mail 4096 2007-05-28 15:59 /var/spool/virtual/domain.tld
chmod 770 /var/spool/virtual/domain.tld
So it is a permissions issue after all.
Code:chmod 770 /var/spool/virtual/domain.tld