IMP already has this capability. You need to talk to your hosting 
provider if they haven't enabled it.

I use your email system on the httpcity.com site.  With the vast 
number of spam hits, I see a potential in your software not in the 
other two offered.  You have a filtering system that is excellent.  If 
you could offer a selection on the mail reading area to select an 
email and place it on the blacklist or whitelist by selecting the 
email and then in the drop down select blacklist or whitelist.  It 
would improve the users ability to put spam in its place.

Currently the user must write, physically write, down the email 
address and then go to the filter and re key it to add it to the list. 
  This addition alone would be a massive help for those of us that 
have been over whelmed by spam.