sed prepend text ?

Hi,

I have a file with email header information. I would like to change the Subject line.

Subject: ** PROBLEM Host Alert: server.domainname is DOWN **

I'd like to change this line such as to look,

Subject: serverID ACK Fw: ** PROBLEM Host Alert: server1.domainname is DOWN**

How can I edit this? sed or anything else is fine.

Note: server1.domainname, is not always constant it could be server2, server3.domainname up until server30.domainname

sed 's/Subject: /Subject: extra stuff/' input > output

Thanks!