Mail::Box, Maildir and the right way to find a message

I have my mail in Maildir format and the folders have thousands of messages.
I want to find all the messages which have been moved into this folder 
since the last time I checked so I can train my spam filter.
The spam filter will want to change the

One way is to open the folder, trundle through each message, and compare 
each messageID with a list of old IDs. 
This seems slow.

Another way, is to keep a list of all the files in the folder and see 
which file are new to the folder and by-pass Mail::Box altogether. 
I'm concerned about multi-session access.

My current idea is to convert the file name into Mail::Box index and 
access it that way.  Is that enough locking?

What is the right way to do this? 

Advice is always welcome.
-Gaylord

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