Blocking MXs by their hostname #383
Description
Here is a simple feature request for consideration.
We would like to block MXs by their hostname.
I did find the CIDR based blocking from the docs:
https://docs.kumomta.com/reference/kumo/make_egress_path/prohibited_hosts/
I also dug KumoMTA source code a bit and I did not find any hostname based blocks. Only the blocking which happens after resolving MX hostname.
In our case this is not adequate because the MX resolves to a so large pool of IP addresses that we'd like to block the hostname.
This way we do not need to begin to maintain and reverse engineer their pool of IP addresses in our config.
One alternative we considered is to use /etc/hosts but this would not be very practical as we would not like to escalate SMTP related confs to the whole host and host configuration.
In an ideal world we would also like to define the status code and the message when email bounces due to blocked MXs so that we can configure our bounce ingester more precisely.
Maybe even have a placeholder in the status message which would be replaced by the MXs that were considered for the email.
We have also observed that the MXs can change quite often with certain type of poisonous MXs.
E.g. MXs which try to hijack sensitive emails by registering typo domains and then collecting corporate emails sent to those typo addresses.
It would be easier to maintain a list of blocked MXs instead of blocked IPs in the KumoMTA config.