Using alias

I have a main address which is my login, It works fine as an email but if i try to use the alternative identities they dont work at all as email addresses, not for sent nor for receive. How can I use the alternative identity for email?

In order to prevent email spoofing and identity theft we do not allow any identity you dont own.
However we also provide email aliases for those who are our regular contributors:
check https://disroot.org/en/services/email