So, yeah, only tangentially related, but if anyone at Anthropic would see it fit to let Claude loose on their DNS, maybe they can create an MX record for 'email.claude.com'?
That would mean that their, undoubtedly extremely interesting, emails actually get met with more than a "450 4.1.8 Unable to find valid MX record for sender domain" rejection.
I'm sure this is just an oversight being caused by obsolete carbon lifeforms still being in charge of parts of their infrastructure, but still...
a not really related fact. I remember reading some RFC, and the sender should try sending to the server specified in A record if there are no MX records present
This sounds like it's an inbound check, as part of spam prevention, by seeing if the sending domain looks legitimate. There are a whole lot of those that are common that are not covered in RFCs.
You are correct that is the expected order of operations