Authentication Failure in Zoho Mail

Hi,

For some reason I've started receiving this message in zoho mail

'Authentication Failure: Kindly change the Mail settings and re-enable auto fetch'

I've tripled checked the server settings but don't see anything wrong. I can still send email from zoho mail but not receive it.

I'm using a hosted exchange account for the mailserver -it worked fine until a few days ago!

Thanks