[exim] Authenticated sender in Received: ?

Hopefully a quickie. I can probably just about manage to change the
received_header_text to include a mention of who the authenticated
sender of a message is, which is what I want to do (both for local users
or messages submitted with SMTP AUTH), but I was wondering if there was
an approved, standard way to do it?

Frankly what I can't quite get my head round, from working from the
default setting, is the rationale for where the newlines do or don't
appear (I know they don't matter but they must be there for a reason)...

Cheers,

John.