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Helping out people with their shared hosting problems inevitably runs into mail issues. One of the more common ones has to do with being put on an RBL, usually a code 550 bounce. This usually happens for one of three reasons:
You'd think that this cross-referencing of a domain's A record to it's PTR would prevent a lot of spam, as this would effectively prevent spoofing. Unfortunately the reality of shared hosting, IPv4 scarcity and limited ISP tooling has crippled this. This has resulted in a reality where there are only 2 correct configurations:
So why is it that we can't share IPs and HELO per-domain/ip? It's precisely because we are doing this PTR to IP to A record cross-referencing as an automated process over at the RBL makers like spamcop et al. That, and almost nobody is ever given a /24 block of IPs (0-254 on the least significant byte of the address). Instead the ISPs usually assign smaller blocks to multiple customers. This means that they can't delegate the NS record for *.X.Y.Z.in-addr-arpa (where XYZ is the remainder of your IP, but backwards) to their client.
So instead they provide some kind of interface for their clients to add PTR records. Unfortunately many don't know that a one-to-many PTR relationship is entirely supported by DNS, much like for A records themselves. As such these interfaces inevitably only allow one domain to be specified per IP. Which leaves you with only one option: send out mail from your server's hostname and primary IP.
Meanwhile, 0 spam is prevented because of this massive hole blown in the entire scheme. The only practical outcome is that unaware sysadmins get caught up by this and are put on RBLs erroneously. Which is a pity, as if the system actually worked correctly it would be an ironclad guarantee against spoofed emails.
IPv6 could have fixed this, as we could give everyone a /64 IP range until the end of time and never run out. Delegating NS for the PTR domains could then become standard practice. IPv6 never really got adopted by the major ISPs though. Given they haven't updated their tooling for multi-PTR (which has been supported almost since the beginning of DNS), we shouldn't hold our breath that NAT is going away anytime soon either.