Mail, is primarily only blocked when there are continous attempts to overload our mailservers or to break in. With junkmail being at 97%, it seems the junkmailers are certainly trying to kill off email alltogether.
The primary reasons are from :
Infected VPS servers
Servers relaying for infected machines
Infected software / scripts
Attempts from the same IP to login to independant mail servers across 6 or more classes
The Solution is simple :
Fix the problem - as you are loosing Mail credibility
Get your provider to fix the infected servers.
Request the block to be lifted from the IP / IP ranges