[exim] Archiving Email

What is the preferred format for archived email?
Archived being just a stored copy for potential legal reasons.

One can use flat files or maildir etc...

But since this email does not belong to anyone, I was thinking, how
would be the best way to go about getting to this stuff in case it was
needed.

With flat files, your friend would be grep. With maildir you would need
a mail client, which might be ok, unless we are talking hundreds of
thousands of messages.

regards, Joseph