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 ..

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Sending a mail to mafkees@siton.nl

root@mx.siton-suzenaar.nl:/data/vmail# echo bla |mail mafkees@siton-suzenaar.nl

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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

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Now I send a mail to pietje@siton-suzenaar.nl
root@mx.siton-suzenaar.nl:/data/vmail# echo bla |mail pietje@siton-suzenaar.nl

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User pietje has UID 10001. Therefor the directory /data/vmail/siton-suzenaar.nl/pietje 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