Editing Email Subject Before Redirect

Can I edit incoming email subject before the redirect? I currently have a catch-all email address setup. If an email address in my domain does not exist, redirect to my Gmail account. I configured my Gmail to add the label "INVALID" to any redirected emails: Gmail Filter: If sent to *@mydomain.com label INVALID unless it has these words (list of email addresess I created on my domain) My Gmail filter is so long now. Gmail is preventing me to edit it anymore. What I ended up doing is creating smaller versions of the same filter, but limiting the email address list to about 15 each filter. This broke the filter altogether. Gmail labels anything coming from @mydomain.com as INVALID now. I was thinking if I can have cPanel check if the email address exist first before the redirect, I can edit the subject line to append "INVALID" at the beginning so it's easier for me to filter it when it arrives to my Gmail. If there is no option, do you guys have any suggestions for a workaround? Thank you.