Milter-greylist

>I don't know: what real MTA will change the case between two delivery
>attempts of the same message? In what situation did you experienced the
>situation above?

I don't think those entries are for the same mail. I think those are
different mails but sent to the same rcpt, from the same user, but with
different (case sensitive) addresses to the rcpt. (For instance, one
message may be a reply on a sent mail from the rcpt, and the other may
be written manually by the user.)