"X-Original-To:" header in email messages -- missing in some messages

Hi all,

I am noticing that Dreamhost has started adding a “X-Original-To:” header in my incoming email messages – this is copied over from the SMTP envelope “Rcpt To:”.

This is great.

However, I am also noticing that this header is being added only to some messages.

For the other email messages, there is no such header.

Can someone enlighten me as to why some of my incoming messages have this header, while some others don’t?

( I have not been able to figure out a “pattern” as to which ones have this header vs which ones do not.)

Thanks in advance.