Custom domain email sometimes marked as invalid?

I’ve recently been working on de-Googling and part of that has been setting up an email with my custom domain. This mostly works great, but one issue I’ve noticed is email validation on some websites detect this email address as invalid. For instance, if I have the domain [name].rocks with the email [name]@[name].rocks (with [name] being a placeholder for my name) my email cannot be used to register with the Ventra app (for getting mobile train tickets) I believe because any site that has an extension with more than four characters is detected as invalid.

I understand this is a validation issue on the end of the app dev / website, but I was wondering if people had suggestions for workarounds when they encounter this? Setting up other custom emails with forwarding? Thanks!