Ah, I just checked my records. I contacted them in 2021-02. Their response was:

> We prefer customers to attach email addresses from widely recognized providers for security purposes. Additionally, our servers have a hard time communicating with private email servers.

> You may create a new email address to handle your tax returns through FreeTaxUSA with a major provider.

thanks for asking them – it's really important to give feedback. Otherwise they just continue to not know.