That said, our security team and I agree there is no security issue here. Mailgun already can see the text of the emails we send.
And again, I wasn't saying that you can't do all of this nonsense, but users who see it as nonsense should be able to turn it off.
The attack wasn't that the attacker has my second factor, the attack was that the attacker tricked me into verifying a single login/transaction using my two factors, on their behalf.
They probably judged that the inconvenience of the verification email affects few enough users that it is worth it. Most users don't switch IP addresses very often. And those that do, probably don't all clear their cookies after every session.
Adding SMS in addition to email would be obviously useless, as you point out.
Even if they were, almost all email goes through third parties which are trusted implicitly. That's not great, but email is the only federated system in existence capable of implementing this type of decentralized login at scale.
Maybe someday we'll be able to use something like Matrix, Fediverse OAuth, or ATProto OAuth instead, but those are all a ways off.
The vendor might not be the only party to use an HTTP redirect service too! My email goes through a security screen by $EMPLOYER, which also rewrites links to get processed through their redirect service. Sure, it's for company-approved reasons, but it's still another party that has access to the login token.
Authentication tokens (even tertiary ones) usually are supposed to have pretty strong secrecy guarantees. I've done multiple security audits for SOC, PCI, HIPAA, etc., and in every case the auditors would have balked if I told them signin tokens were being unnecessarily logged by a third-party service.
(Also: I strongly disagree that the only way to get reliable delivery is via a third-party email service, especially at Mercury's scale, but that's a digression from the topic at hand.)