Change subject of Email received

We are now using QuickMail of Outspring and do not want to change to Mail, mainly due to the following problem of Mail:
Many of the Emails we receive do not reflect the keywords we use internally in our company, such as ordernumber, enquiry number, etc.. In Quick Mail we can easily add that number to the Emails received and file accordingly. Consequently we can easily find all correspondence on the relevant internal keyword/reference number by just typing that reference in the search machine of Quick Mail.

Any Mac Mac OS X (10.4.3)