su makes not proper root

[Zoran Kolic <zkolic%sbb.rs@localhost>]

On raspberry pi I installed 7.99 and use
su to get root. At first, I spoted that,
during shutdown, message says it came from
zoran@localhost. I expected root@localhost.
Now, I have a lot of problems configuring
mail app. I could send mail outbound via
sender_canonical_maps, send from root to
user correctly. If I send from user to root,
it goes to /var/mail/root file, but mail
tries to read /var/mail/user file. I might
change it with "folder /var/mail/root" and
read as I should.
I'm puzzled how su on netbsd works. Have I
made a mistake or it is normal behaveour?
If normal, how to set /var/mail/root, since
.mailrc > "set folder=/var/mail/root" does
not work.
One thing to stress. I login as user via ssh.
The only option to get root is su, obviously.
Best regards

                       Zoran