Looking for an Addin to show client side banners - Outlook 2016

Hello there,

for quite some time I've been looking for an Addin to display something like banners or text. The catch here is that it has to be client side. It should not alter the html code of the email in any shape or form. 

I need it to display a small text on incoming emails from extern (simple html would suffice here) but due to the way the anti spam works I use, it would give me a ton of false positives.

Always removing and then adding it again, before it passes the anti spam would ruin email signatures, thus it has to be client side and not altering the mail body.

If someone of you knows a workaround for this problem, or an Addin which could get the job done, I'd be really gratefull.