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 machines 

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