How Do Local Privileges Affect the Ability to Add an Attachment?

[nb:i posted this earlier under a different heading, but am reposting because
the focus of the issue has changed. i noted on the previous post that it can
be ignored.]

outlook 2k3 sp3, exchange 2k3 sp2

when attaching a 21mb file to an email in outlook, i get the following
message:

"Microsoft Office Word
The attachment size exceeds the allowable limit."

- the error occurs when trying to add the attachment to the email, NOT
after sending the email with the attachment already in place (which is
how i've experience this kind of error in the past)
- the user's profile 'delivery restriction' is set to use the default limit
- the exchange server default limit is set to 50mb (as set in the Exchange
System Manager - Global Settings - Message Delivery - Defaults). the
error occurs even when the default is set to 'no limit'

most interestingly, i found a work-around that would seem to indicate where
the problem lies:

if i make the user an Administrator on his pc, then he can add the
attachment. however if he's a Power User (as is everyone in our company), he
cannot add the attachment. which begs the question:

how do rights/permissions come into play when adding an attachment to an
email?

thanks for your help,

ross