Unable to send email or reply to messages (relaying disallowed).

Unable to send email or reply to messages (relaying disallowed).

When sending email (I'm using POP, with Mozilla Thunderbird as a client) I cannot send email message or replies.

The error which appears states "An error occurred while sending mail. The mail server responded:  Relaying disallowed as J Wheeler < j.wheeler@zoho.com>. Please check the message and try again."

Please can someone tell me why I'm not allowed to send mail, and if this can be fixed?