Are tere any news if (and when) Exchange Online plans to support RFC 7505 ?
RFC 7505 allows to define explicit null-mx record.
Gmail is already supporting this.
When I email a domain with null-mx record I instantly get this nice looking NDR:
Delivery to the following recipient failed permanently:
Technical details of permanent failure:
DNS Error: 33796847 DNS type 'mx' lookup of domain.com responded with code NOERROR
The domain domain.com doesn't receive email according to the administrator: returned Null MX https://www.rfc-editor.org/info/rfc7505
Email sent from Exchange Online will go into 'normal' pending/retry mode and after 48H generate standard "host unreachable" NDR.
It appears not te be planned, as I could not find it on the roadmap nor on https://office365.uservoice.com so my suggestion would be that you put it on https://office365.uservoice.com and have some voters on it to get it to the attention of the engineering team. My experience is that Microsoft watches this closely so it should help you get this feature into Exchange Online.