I wouldn't be surprised if that's intentional. There's an explicit hesitance on the part of mail providers to accept v6 mail, since they use IP addresses as a reputation mechanism. IPs that originate spam mail get summarily executed, and getting new IPs that have a high antispam reputation is actually quite expensive.

In other words, it's a Sybil-resistance mechanism, called Proof-of-IPv4. It works specifically because v4 addresses are scarce. v6 addresses are not nearly as such. Everything that makes IPv6 great for the Internet at large makes it terrible for mail providers. For example, because the original v6 design wanted to eat lower link layers, it reserves half the v6 address for an embedded MAC64. This never really panned out, but it's terrible for security, so every v6-capable OS nowadays will rotate addresses every few hours. The average machine will have hundreds of addresses. How do you assign a usable notion of per-IP reputation to that?

You could use v6 subnets for reputation, but there's still 64 subnet bits - enough to stick an entire IPv4 subnetwork inside of each IPv4 address. Some ISPs actually will assign a /64 per customer (because Comcast needs something to sell to Business customers), while others assign /56s or /48s. So there isn't even one granularity of subnetting that you can use for reputation tracking on v6.

Meanwhile, v4 pricing is getting worse and worse, which is great for mail providers. They don't necessarily need to turn a profit on incoming mail, but they do need to make it expensive for people who want to send lots of spam.