random/occassional "permission denied" failures in "sa-update"

[snowcrash+spamassassin <sc...@gmail.com>]

at seemingly random intervals, sometimes after days of working just
fine with no errors, and with no extraordinary actions on my part,
"sa-update" will fail with:

channel: attempt to rm channel cf file failed, attempting to continue
anyway at /usr/local/spamassassin/bin/sa-update line 742.
error: can't remove file
/var/mail/spamassassin/updates/3.002000/updates_spamassassin_org/10_default_prefs.cf:
Permission denied
channel: attempt to rm channel directory failed, attempting to
continue anyway at /usr/local/spamassassin/bin/sa-update line 745.
error: failed to open
/var/mail/spamassassin/updates/3.002000/updates_spamassassin_org/10_default_prefs.cf
for write: Permission denied at /usr/local/spamassassin/bin/sa-update
line 993.
channel: archive extraction failed, channel failed
error: can't remove file
/var/mail/spamassassin/updates/3.002000/updates_spamassassin_org/10_default_prefs.cf:
Permission denied
channel: attempt to clean up failed extraction also failed!

suggestions as to why this is happening, and what to do about it?

thanks.