Access-by-email doesn't seem to work for our domain

It appears a hyphen is not allowed in the domain for the access-by-email feature. If I enter @myexample.com it works. If I enter @my-example.com, I get this error:
Sorry '@my-example.com' doesn't look like a usable value for access-by-email.

The problem is, our most-used company domain has a hyphen in it.