[anti-abuse-wg] Mimecast.com

Hello,

Has anyone of you had much/any dealings with this crowd: Mimecast.com ?

Less than 1% of our email volume exchanges with them, and yesterday
after complaining about abuse/UBE from them and receiving no response,
escalated to @telstra and then all of a sudden they are returning all
email in a loop:

  support at mimecast.com
    host service-alpha-inbound-b.mimecast.com [91.220.42.231]
    SMTP error from remote mail server after end of data:
    554 Email rejected due to security policies -
    https://community.mimecast.com/docs/DOC-1369#554

Then, when I email them from a different server, it goes through the
first time and viola, the second email, same block, on the "new" server"

From their website: They provide email filtering services, this feels
like they are extorting money from me, which I will not pay of course.

But, have any of you had any similar experiences? As they are
providing this type of "service" to some of our @gov departments
as well as other large companies, I will have user blowback - what
do I tell my users? We are blocked because we dared complaining? 
or we are blocked because we do not pay?

any advice will be appreciated, obviously this type of behavior breaks
email and the larger the "protectors" become, the more control they
have...

tia

andre