[Exim] Calling exim as a user with no entry in /etc/passwd

Hi,

I am currently in the process of finishing an implementation of a virtual
hosting system where each user has a unique UID and can run CGI scripts
under that UID. Rather than create individual entries in /etc/passwd for
each user, I have just assigned unique UIDs and set permissions accordingly.
This works great for everything, except for when a user tries to call exim
to send an email from a CGI script (exim -t). The following error comes up
on stderr and in the paniclog:

2000-04-12 16:23:35 Failed to get user name for uid [xxxx]

where [xxxx] is the numeric UID of the user calling Exim. Is there any way I
can tell Exim not to worry about the lack of mapping from numeric uid to a
name? I really don't fancy adding a few thousand entries to /etc/passwd just
for this. Potentially there will be many users on this system (does anybody
know the maximum UID FreeBSD 3.4 will allow? Is it 64K or higher..?)

Thanks for any pointers,

Jonathan