Many, but Many spam/virus emails been sent from a blocked domain

[m0rpheu5]

Hello guys, i´m having problem, i got an issue, that my server got blacklisted, so i looked for the problem, and i found many, but many email been sent from an email that doesn´t exist from a domain that is blocked.

Like, there is many email been sent from Comunicado@myclientsdomain.com and this domain @myclientsdomain.com is blocked in my plesk, and the email service is blocked too.

So how this email are been sent??

How can i block this??

Thanks