I sent an email invitation to join my forum using the "Invite friends" form. My friend received the message and tried to answer, and got an error message: "Delivery to the following recipients failed: board@forumativo.com".
Maybe I did something wrong? Please help me to solve this problem

He can't reply to that email address because it isn't an actual reliable email domain. Its an automated message and you can't reply to them.

Thank you. I thought the replies would be forwarded to the admin. Then I must tell them not to reply, and give another contact address.

You could always create an email signature via the ACP that warns people not to reply to that address and provide an alternative but remember, everyone would see that email address.

Is this solved?

Yes, it's solved. Thanks for your help.