You did not give me to work with but let me at least break down what the error means for you.
That looks like an error message returned by another host when trying to send mail to iit. Are you trying to send mail from a dynamic IP or an IP for a DSL or Cable connection using your own mail server? Some hosts won’t allow connections from these IPs; to get around that problem you may have to gateway your mail through your ISP. A 553 can also come from a misconfiguration on a client , an incorrect email address or it can also be that your domain information has not propagated yet.
Did this problem just pop up? Have you made any changes? Can you reply with your setup?