Insert a message for outside Emails?

I am looking at various sieve folders and wonder if anyone tried to 
create a filter that inserts a message like "Email from Outside Adresse" 
at the beginning of each message body, if the sender domain does not 
match the local one.

Is that possible?

I am getting my emails via fetchmails -> postfix -> cyrus, so something 
in postfix itself would not work.

Cheers

TB


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