Different mail size : attachment and inbox (size)

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Hello,

There is a strange problem using outlook 2003 (exchange 2003).

Sometimes, when I received an email with attachment (in pdf), if the size of
the message in the inbox is very different of the message itself.

Examples:
- we received some docs, the sum of the document was about 600 kb, but the
email counted for 2 mega ! I did the "editing" trick and back to normal.
- I received an email with ad attached zip of 3 mega, but the email counts
for 4; I then just "flag" the email and it was reduced to 3.
When the message is edited, and one character changed in the subject or
anywhere the size become "normal".

You can understand the inpact on the size of mailbox of this bug, can
something be done?

Have you ever seen this problem ?

How to solve it ?

Thanks