To fix this, I tried to run spamd as the user `mail'. However, when I try that, I get the error that 'spamd does not have permissions to write to /root'. How do I get spamd to know that mail's home directory is at /var/spool/mail.
Set mail's homedir to /var/spool/mail in your passwd file. Spamd just picks up the homedir from what the OS tells it.
Is it a good idea to run spamd as mail?
That's somewhat reasonable, although it would be a little bit better to run it as it's own deprivleged user.