SMTP Error: Mailbox unavailable. The server response was: User not authenticated

SMTP Error: Mailbox unavailable. The server response was: User not authenticated

Hi Zoho Staff,

We have encountered the following error for the past 2 days while trying to send email using the Zoho SMTP settings.


ERROR: Mailbox unavailable. The server response was: User not authenticated


For many months the settings have worked very well.  We have NOT changed the settings.

Do you have any insight as to why this is happening?

Can you help fix this error?


Thank you,

Larry