No. One directory per date, one file per message named for the time (to the minute; hence the 1440) of the action + disambiguator. This was based upon the assumption that your "several hundred thousand" figure would be spread across several days.

20051103 - 0502.xxx1
           0502.xxx2
           0503.xxx1
           0503.xxx2
           0505.xxx1
           0506.xxx1
20051104 - ...
[download]

If I was misreading you, and your mean several hundred thousand per day, and daily directories would be too big, then I'd sub-divide them into hours

20051103 - 00
         - 01
         ....
         - 23 - 5901.xxx1
              - 5901.xxx2
              - 5902.xxx1
[download]

I guess you could go to minutes, if required to keep the final directory sizes small for searching. Having 1440 subdirs in a directory isn't a problem as you are never searching that space, just going through it with a hard-coded path.

I'd stick to one record per file, to avoid the need for using (cooperative) locking, as it only takes for someone to use a non-cooperating utility at an inappropriate moment to create a dealock situation.