Null MX is for positive signalling: https://www.rfc-editor.org/rfc/rfc7505.txt https://www.rfc-editor.org/rfc/rfc7505.txt For domains previously sending mail b

Null MX is for positive signalling: https://www.rfc-editor.org/rfc/rfc7505.txt https://www.rfc-editor.org/rfc/rfc7505.txt For domains previously sending mail but no longer (transitioning to a new domain name), DKIM selector revocation is basically a cleaning-up (hinted at "Revoke all existing DKIM selectors in both TXT and CNAME records."). This prevents abuse at servers who don't understand DMARC but might be fooled by an errant DKIM-signed message (especially old 512-bit keys, I know key rotation but considering this is intended for government consumption there will be old systems still using a 512-bit RSA DKIM keys). This is just brownie points however if the domain never sent a valid mail at all.