How to restrict internet attachment

Hi there,
I run E2K with SP3 on W2k also with his SP3.

All of our users can send internal emails with or without attachements.
However, my boss asked me to restrict some users from sending internet email with attachments but the message itself has to pass through.
On the other side, those same users can send the same message with attachments internally.

I already have one SMTP connector to restrict other users to send internet emails.

How can do it???

Christian
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