mailserver: remove ix.dnsbl.manitu.net blacklist

[osnyx]

Has been shut down, now causes all incoming mails to be rejected

PL-133519

@flyingcircusio/release-managers

Release process

• Created changelog entry using ./changelog.sh

PR release workflow (internal)

• PR has internal ticket
• internal issue ID (PL-…) part of branch name
• internal issue ID mentioned in PR description text
• ticket is on Platform agile board
• ticket state set to Pull request ready
• if ticket is more urgent than within the next few days, directly contact a member of the Platform team

Design notes

• Provide a feature toggle if the change might need to be adjusted/reverted quickly depending on context. Consider whether the default should be on or off. Example: rate limiting.
  • hotfix, in case we add a new list that should be configurable via NixOS options
• All customer-facing features and (NixOS) options need to be discoverable from documentation. Add or update relevant documentation such that hosted and guided customers can understand it as well.

Security implications

• Security requirements defined? (WHERE)
  • availability: hotfix the accidental rejection of incoming mails
• Security requirements tested? (EVIDENCE)
  • manually tested on a mailserver that this unbreaks incoming mails