That's just one current example of when email delivery matters, likely to resonate with HN as being senders and receivers they recognize struggling with this. Not just a Microsoft thing, we can talk about a dozen where Gmail files things wrongly as well, and where the common element is the mailer.

These mailers all have different levels of trust and deliverability stats. It's critical to know.

> That ("Anthropic's" deliverability issue) is a Microsoft issue.

Emphasizing this. Validation emails sent to Microsoft hosted email are at high risk of never arriving.

For at least 2 years I've seen this on MS's free, paid and enterprise/hosted services. The issue is per validator; mails all arrive or none ever will. Generally I'll try again months later and find that service is still blocked.