trisquel-usuarios doesn't allow some subscribers

I'm trying to subscribe to that mailing list with my openmailbox.org account, but upon inserting my email, name and password: my browser tells me that the next page is insecure, and then I accept, and so when I arrive there I see the following message from the mailing list server:

Trisquel-usuarios Resultados de la subscripción

Debe proporcionar una dirección de correo electrónico válida.

It's telling me that I must provide a "valid email address".

Some time ago I once tried to contact the listadmin directly, but got no response. Now I decided to try again thinking that the solving of current issues would also solve this problem, but the issue persists, so I'm trying contact here instead.