Why do some files in dir list have appear in red & have '?' instead of the file properties?

Why do some files in dir list have appear in red & have '?' instead of the
file properties?

I've just found that in at least one user's Maildir some of the files in
the cur directory appear 'flashing'(!) in red in my terminal and the same
files have their properties/attributes replaced with question marks.
I discovered them because I ran du -hs on the home directory and those
files were listed as inaccessible even to root.
Here is the listing:
drwxrwx--- 2 travel mail 8.0K Aug 11 19:29 .
drwxrwx--- 5 travel mail 4.0K Aug 11 19:30 ..
-rw-rw---- 1 travel mail 33K Jun 27 03:55
1327311651.M313310P1633.my.domain.info,W=2330:2,STa
?--------- ? ? ? ? ?
1327378561.M795425P7819.my.domain.info,W=4603093:2,ST
?--------- ? ? ? ? ?
1327408110.M597988P17812.my.domain.info,W=3910:2,ST
?--------- ? ? ? ? ?
1327665200.M938383P29773.my.domain.info,W=5762:2,ST
?--------- ? ? ? ? ?
1327904625.M697896P18082.my.domain.info,W=7674:2,ST
The machine is a VPS running CentOS 5.