Keyword based custom subject warning

Hello,

apparently, our users are receiving emails containing links and /or 
references to various publications, some of which are considered 
'predatory'. Emails usually contain ISSN (International Standard Serial 
Number) and the idea is to scan the body of an email for the ISSN and if 
it matches one on the 'blacklist', the subject of the email should be 
prepended with some specific warning (other than ***SPAM***).

We're already using Amavisd-new on our inbound SMTP server, so I wonder 
if that would be possible with it?


    Thanks,

      Danilo