I've got a recent requirement to carry around a unique id in the email for a customer's server to look for in replies.
It cannot be 'visible' to the recipient.

I've got two options:
1. Hide it - I know how to do that safely and their reply handler picks it up fine.
2. Shrink it - make it tiny and at the bottom so whilst it might be visible, it won't be noticed (font-size:1px; line-height:1px etc.)
(the third was to add it as an html attribute but for various reasons that's not an option)

Hiding it seems the obvious method, however, I'm concerned that some self hosted mailservers and/or inboxes might strip out hidden content when it makes/sends the reply and I cannnot afford not to have it when they get it back.
So unless I can be sure that hidden text won't be lost on the way, I'll have to make it tiny.
If I make it tiny, are there any deliverabiltiy issues? I've never found one in doing it for other things but I thought I'd best ask.

So...

1.Does anyone have an insights into if hidden copy is removed by any inboxes/mailservers on a reply?

2.Does anyone have any insights into very small text causing junking or rejection?