unfortunately it is only a matter of time before altermime (the engine) is tagged as non maintained. Indeed you have no updates no maintenance since 16 november 2008 and you have no alternative for postfix
I see three possible ways
- use altermime alone without amavisd
- use rspamd to add signature (not sure)
- fork altermime and start a new project
Sure that in maybe a short time, your signature script could die :’(