Can't send emails from iMail - error 5.5.1 Authentication Required

Can't send emails from iMail - error 5.5.1 Authentication Required

I cannot send emails using iMail. All other accounts (Hotmail, Gmail etc) set up successfully. Receiving emails with no problem but nothing will send.

Getting error message:

The sender address [xxxxxxxxxx] was rejected by the server smtp.zoho.com.

The server response was: 5.5.1 Authentication Required.

Select a different outgoing mail server from the list below or click Try Later to leave the message in your Outbox until it can be sent.

Have tried everything I can see/think of and even my website company cannot work this error out.

Please help!