At first there is nothing in /data/vmail
root@mx.siton-suzenaar.nl:/data/vmail# ls -alR
total 4
drwxrwxrwx 2 root wheel 512 Apr 9 15:52 .
drwxr-xr-x 4 root wheel 512 Mar 20 02:32 ..
-----
Sending a mail to mafkees@siton.nl
root@mx.siton-suzenaar.nl:/data/vmail# echo bla |mail mafkees@siton-suzenaar.nl
-----
This is how things look when this mail is delivered. Note how the directory siton-suzenaar.nl is owned by uid 10000
root@mx.siton-suzenaar.nl:/data/vmail# ls -alR
total 6
drwxrwxrwx 3 root wheel 512 Apr 9 15:54 .
drwxr-xr-x 4 root wheel 512 Mar 20 02:32 ..
drwx------ 3 10000 wheel 512 Apr 9 15:54 siton-suzenaar.nl
./siton-suzenaar.nl:
total 6
drwx------ 3 10000 wheel 512 Apr 9 15:54 .
drwxrwxrwx 3 root wheel 512 Apr 9 15:54 ..
drwx------ 5 10000 wheel 512 Apr 9 15:54 mafkees
-----
Now I send a mail to pietje@siton-suzenaar.nl
root@mx.siton-suzenaar.nl:/data/vmail# echo bla |mail pietje@siton-suzenaar.nl
-----
User pietje has UID 10001. Therefor the directory /data/vmail/siton-suzenaar.nl cannot be created.
Apr 9 15:56:53 mx postfix/virtual[55004]: warning: maildir access problem for UID/GID=10002/1: create ///data/vmail/siton-suzenaar.nl/pietje/tmp/1144591013.P55004.mx.siton-suzenaar.nl: Permission denied