vexim and procmail with piped command. Processed mails belong to Debian-exim

Hello,

I have a little problem:

My vexim user is "vexim". The virtual mail directory belongs, of course, 
to this user. When I run procmail through the piped command, the result 
are mails in my Maildir with ownership "Debian-exim".

How can I ensure that the files are owned by vexim?

Or am I making a general error in reasoning?

Any hints are appreciated.

Thank you,

Hermann