Mail sent messages mailbox file is corrupted

Yes, that makes sense.

Incidentally, the POP account in question no longer has a Drafts BOX or a Sent Box in the lefthand sidebar. Could that be way to help Mac find things, by making a draft and sending a message? I don't see that it could cause any harm.

I am doing searches of the external SSD clone with my blazing fast MacMini M4 to help determine locations and any losses of emails. I do see that some files are only partial.

The older version from a week ago is a boot clone and I could select what to copy back.

I have used Mail for umpteen years and each new machine transferred everything from the previous model, which used to be every 2-3 years until this machine. (I did not buy a new one for various reasons).

An attempted TM restore messed up things a week ago, but it appears that corruption had started before. Reindexing this particular folder had not worked for some time, which I had attempted because the search function wasn't working well. I have now turned off TM until this is solved.

So there is a long history in the Mail installation with 5 active accounts. My only perceived problem to solve currently is the Sent Box of one account. Everything else had been working perfectly.

"And of course I would be extremely wary of copying or exporting the corrupted mailbox. "

Why do you say that?

The older version from a week ago is a boot clone and I could select what to copy back.

I have used Mail for umpteen years and each new machine transferred everything from the previous model, which used to be every 2-3 years until this machine. (I did not buy a new one for various reasons).

An attempted TM restore messed up things a week ago, but it appears that corruption had started before. Reindexing this particular folder had not worked for some time, which I had attempted because the search function wasn't working well. I have now turned off TM until this is solved.

So there is a long history in the Mail installation with 5 active accounts. My only perceived problem to solve currently is the Sent Box of one account. Everything else had been working perfectly.

Also, the system has created about 8 UUIDs over time & I am currently running "Find Any File" simultaneously on this, the affected machine and on my MacMini M4 (much faster) with the 2 external SSD clones to determine the exact UUID location of my missing mailbox.

"And of course I would be extremely wary of copying or exporting the corrupted mailbox. "

Why do you say that?

I have not yet tried to replace the existing mbox, but have extensively inspected an isolated copy. It is very large and probably the reason for the initial problems. Everything appears to be very tidy and arranged into multiple sub folders and sub folders therefore, ad infinitum, thus the size. The organisation according to typical Apple fashion. There are surely many duplicates but I have not seen a single message which looks corrupted. (I correct my earlier suspicion). 'Corruption' is ambiguous, unless that term includes duplicates.

Is there a paid (or unpaid) tool available to inspect this mbox? Alternatively could I just import it into Mail (I see that function is available). I would select a new destination mailbox name and immediately start with manual de-duplication.

Thanks, I have tried all of that and already been working on this the whole week.