The DNSBL "MegaRBL.net" is no-more. According to the Internet Archive, MegaRBL had been around since at least some time in 2013. It was a non-commercial independently run spamtrap-driven blacklist. Mailop subscribers suggest the list may have been dead for years, but the Internet Archive shows its website being alive and active as recently as March 2019.
As of today, November 25, 2019, the blacklist's domain name appears to have expired and the new owner or domain registrar has implemented wildcard DNS. This has the net effect of "blacklisting the world" and it means that if you use this DNSBL in your mail server configuration, you're likely to now be rejecting all attempts to send mail to your users.
You don't want that! Remove the "MegaRBL.net" DNSBL from your mail server configuration as soon as possible.