Hello.
There should be some problems with postfix, libc6-2.13 and kernel 2.6.37 (or less?).
If you find in the mail log /var/log/syslog several lines like:
Oct 31 07:43:43 gw postfix/master[22311]: warning: process /usr/lib/postfix/local pid 23869 exit status 1
Oct 31 07:43:43 gw postfix/master[22311]: warning: /usr/lib/postfix/local: bad command startup -- throttling
Oct 31 07:44:43 gw postfix/local[23950]: fatal: unable to determine open file limit
then test your mailing system issuing the following command with normal user priviledges:
If you get the following errors
postdrop: fatal: getrlimit: Operation not permitted
send-mail: warning: command "/usr/sbin/postdrop -r" exited with status 1
send-mail: fatal: psubiaco(1000): unable to execute /usr/sbin/postdrop -r: Success
Can't send mail: sendmail process failed with error code 75
then most probably you need to upgrade your kernel to version 2.6.38 or later.
It means that the last C library needs a new kernels to work.
I got this problem in a debian server using postfix with 2.6.37 kernel. Upgraded to 2.6.38, the problem was fixed.