For what it's worth, sendmail's implementation encodes the current time down to the second plus the pid of the handling process in its queue IDs. A collision then could only happen if the same pid got re-used twice in the same second. It doesn't include the inode or any random data.
Details: http://www.ale.org/pipermail/ale/2001-May/022331.html Similar to the issue of log correlation, in the OpenDKIM stats project work we had to have an SQL key across the reporting host, queue ID and timestamp columns to account for the fact that postfix recycles queue IDs, sometimes relatively quickly. -MSK
