> I've been sending my mail via IPv6 for ages without issues…

In most cases it does work and there are no problems. But especially medium-sized businesses who run their own email through some crappy mailfiltering middlebox file anything that comes from or through an IPv6 endpoint to spam.

> did you mean reputation or reputation?

I don't understand your question. I meant IP address reputation, which is what many mail filtering systems use to estimate spam probability or outright reject email. See for example https://www.spamhaus.org/ip-reputation/

That does not match my experience either. In fact I don't see why any mail server would bother getting and configuring an IPv6, listen on it, just to drop all received messages.

Just don't listen on v6, that would achieve your goal, would prevent reputable dual-stack senders (like Google!) to send messages over v6 that you'll drop, and simplify your configuration.

> I don't understand your question. I meant IP address reputation, […]

There's reputation of the IP address(es) among spam filters, and there's reputation among human operators configuring mail servers. I was trying to riff on ambiguity with ambiguity, that wasn't particularly helpful, sorry.