How come that clamscan can't access some incoming emails, mostly infected. See the following report.

WARNING: Can't access file F:\Program\RaidenMAILD\Inboxes\xx\_20070912035506-450907602-3392.eml

----------- SCAN SUMMARY -----------
Known viruses: 152098
Engine version: 0.91.2
Scanned directories: 0
Scanned files: 0
Skipped non-executable files: 0
Infected files: 0
Data scanned: 0.00 MB
Time: 13.369 sec (0 m 13 s)

Scan Started Wed Sep 12 03:00:00 2007

there can be a number of reasons, all unrelated to clamwin itself
- Another on-access AV blocking access
- file being opened for exclusive use by another program (i.e mail server during POP3 delivery)
- lack of security permissions

This happens sometimes, not every mail.
Clamscan is on my mailserver with Raidenmaild.
There is no other AV-program on the computer.
All clients were shut down, so there was no POP3 delivery going on.
If it is lack of security permissions how can I check this?

I think more likely reason is your mail server opening the file for exclusive access. It can be when your mailserver writes the email to a file after during SMTP session