[exim] Sender address against authenticated_id?

Hello,

I am trying to figure out how to figure best, that an authenticated_id
somehow fits to whatever sender is given in the envelop of a mail. What I
find confusing about it is firstly AUTH Parameter in the MAIL command seems
to possibly contain a different email address than given in the FROM. The
example from the Exim Guide[1] on Page 287 is

MAIL FROM:<theboss@???> AUTH=joker@???

If this is set by the sender of the mail and not added by exim, does exim
somehow verify this?

Secondly I am notquite sure how to deal with aliases... for example, several
users may be allowed to respond for webmaster or for postmaster or root. If
Id simply say a user may only use whatever email address is given in
/etc/email-addresses that may be extremely limiting.

Isn't it normal to check for this, somehow? If it is configured in default
Debian (and therefore Ubuntu) configuration I must have missed in by now.
The version I use is 4.63 out of Debian Etch with the matching config
package.

Thirdly I wonder wether it is possible or makes sense to somehow check
From:-headers within the message body...

Regards
Björn Keil

[1] The Exim SMTP mail server: Official guide to Release 4, Second Edition,
copyright 1997 UIT Cambridge by Philip Hazel