I wasn't able to sign up for postmaster@rootshell.is, but I was able to get abuse@rootshell.is. You should be careful about what standard email addresses you allow people to take. I recommend you take abuse@ back from me and you should really have a strong denylist. I just asked an LLM for a list of things you should be blocking and it came back with the following. The cert validation ones seem particularly important:

RFC 2142 mailbox names (the core list):

postmaster@ — required by RFC 5321; mail systems expect it to always work abuse@ — for reporting spam/misuse hostmaster@ — DNS issues webmaster@ — website issues noc@ — network operations security@ — security/vulnerability reports info@, marketing@, sales@, support@ — business functions

TLS/certificate validation addresses (RFC 8552 / CA-Browser Forum):

admin@, administrator@ ssladmin@, ssladministrator@, sysadmin@ These can be used to validate domain control and issue certificates, so handing them to a random user is a real security risk.

Common automated/system senders people impersonate or that cause confusion:

noreply@, no-reply@, donotreply@ mailer-daemon@ — bounce messages (RFC 5321 sender) root@, daemon@, bin@, sys@ — Unix-style system accounts null@, devnull@

Brand/trust-sensitive ones worth blocking too:

billing@, accounts@, payments@ help@, contact@, service@ legal@, privacy@, dmca@ register@, registration@, signup@ The service's own name (e.g. [brand]@, team@, staff@, official@)

[edit] Re the TLS issue. You should set up a CAA DNS record and also check on crt.sh later to see if anybody managed to get a cert for rootshell.is if you didn't lock down the validation addresses

Wouldn't the better guidance be to use different domain for official communication similar to sites where you can customize the subdomain? Attackers can always come up with something you didn't think to block.

Google doesn't let just anyone make a mail on the google.com domain for example.

I found the guy on X, wasn't that hard: https://x.com/haptagod You should probably hit him up and tell him these things?

I hate shoving LLMs everywhere, but honestly this is probably a good use case for tiny models like the 0.6B Qwen model to flag account names for human review.