Much clearer. It may work, but even if it does it may be problematic. I would be inclined to investigate alternate platforms that were free of the mailbox constraint before pursuing a common incoming mailbox much further.

Some issues you might think about:

Will the original envelope recipient be recorded in the catch all mailbox and available to mail-fetch.cgi?

For example, sendmail can record the envelope recipient in the Received header, but not all MTAs record this information. For various reasons (e.g. BCC, mailing list expansion, etc.) the envelope recipient may not be recorded in the body headers.

If you don't have access to the envelope recipient address, how will you determine which of your users should be able to see a message? I am doubtful that there is a general, secure and correct solution to this problem.

What happens if an incoming email is addressed to several of your users? Will it be replicated into your catch all mailbox - once for each user? If not, what happens when one of the recipients deletes the message? How will your server know when all recipients have deleted the message?

Consolidating messages to multiple users into a single mailbox will exacerbate the impact of capacity and performance limitations of the mailbox format.

You may have concurrency issues accessing and updating the single mailbox, depending on its form. If it is a common mailbox file with on the order of 1000 busy users, this will almost certainly become an issue.