permission problem with os.setuid

[Michele Simionato]

I have a script that sometimes is run by myself (user id 501) and sometimes
by the mailer program as nobody/nogroup (userid 65534). I would like to change 
the effective uid to 501 in any case, to get the right permissions, but
os.setuid and os.seteuid give me a OSError No. 1. Is there a way to get what I
want? My requirement is that the script should work indipendently from the
mailer program, i.e I would like to avoid configuring the mailer program by
hand. I am working on linux with postfix on Mandrake and exim4 on Debian.
Any suggestion?

        Michele Simionato