User Unknowns .. If user is a number with Sendmail + Procmail

[Sonny Kupka <sonny@nothnbut.net>]

I have a major problem I'm trying to debug..

I have couple users that have username of just numbers..

8400 is one case.

You can finger 8400 it's there
You can grep for 8400 in password file and shadow file and user is there
/home/8400 is there

Before switching from Slackware to Debian user could get mail now his mail 
is bounced out user unknown.

Anyone have any ideas what to look at?

Thanks!

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Sonny