check_policy_service sends size=0

[dovecot at ptld.com]

> It sounds like a better approach would be to make Dovecot reject any
> email no matter what the size is (even size=0) if a user is over
> quota. I'll take further discussion to the dovecot mailing list then.

If you are not aware, dovecot has by default a 10% over quota grace. So 
even being a couple of bytes over quota it would still accept a small 
message.