Mimecast at many universities is blocking emails from the AustMS21 registration system. This ranges from annoying to catastrophic (e.g. when password reset emails get blocked).
There is something people can do, however: they can log on to the Mimecast portal at https://login-au.mimecast.com/u/login/#/login with their university email. There, they can release held messages and permit certain addresses (such as firstname.lastname@example.org) and domains. This might be useful well beyond just receiving emails from the Conference registration system.
Florian (AustMS2021 conference director)