[mynameisvlad]

Companies blocking disposable email domains are doing it to prevent many users from using their service with a disposable email.

A custom domain that only I use specifically for disposable emails would look indistinguishable from any other custom domain out there, and nobody else would have used it for them to even be aware of its existence.

To block it pre-emptively, they’d have to either be omniscient or block every single custom domain in existence. The former I highly doubt is the case, the latter would generally do more harm than good to them.