[exim] Spool error: permission denied on Mac OS

Hi,

I'm trying to setup my own exim server on my old mac mini so that it can
send and receive email for me.
I search various sources and then come to the point that I get one error in
the log file of exim.
The error I get in the log file is "
Spool error for /usr/local/var/spool/exim//input//QDWI9R-000ZKP-FT-D:
Permission denied"
I tried to enable all debug information in exim daemon.
But I did not find the required permission and the user from whom the
process is based on.
Using "ps" command, I can see the user is macports.
The email is a test email which I send to myself using the same email
address.
But if I use exim -qff -v, I can see the email are flushed into the
destination email box.
So my question is if there's a way to solve the permission denied error so
that the email I send out can go directly into my own mailbox?
One thing seems strange to me is I can find all -D, -H, and -J file created
for each email I send out.
But the error is coming from spooling the -D file.

Thank you for your attention and look forward to your reply.