If you use any of the MSRBL DNSBLs, take note: For the second time since 2017, the domain msrbl.net has expired and its name servers are responding positively to any DNS request.
This has the net effect of the DNSBL "blacklisting the world." If you use any MSRBL blacklist in your mail server, you're blocking all mail from any IP address in the whole world.
So....don't do that!
The MSRBL's website at msrbl.com is up and running, but the DNSBL zones are not under "dot com" -- they are under "dot net."
June 14, 2019 Update: Looks like the DNSBL has been restored and is no longer "blacklisting the world."