Create a Hash Ring For Each Allowed Domain in the Email Distributor
This will isolate the bridges such that if one provider is subverted it will not have the ability to obtain all bridges assigned to the distributor.
The first easy way to do this is to change the EMAIL_DOMAINS config option to accept (domain, proportion) pairs instead of only the the domain name. Adding a new config option specifically for this will also work, but initially it seems unnecessary.