Spam Leading to Getting Blocked

Hello,

I recently set up my family on mailbox.org and one of the family members have had their email address spoofed to send hundreds of spam messages. Since this occurred, they have been unable to receive or send any emails. Upon trying to send them an email, I get the following error returned back to me:

SMTP 550 - User unknown in relay recipient table

Before this spam occurred, their email was working fine. Any way I can fix this?

Thanks