I was working for client who uses Zoho for their email service. When I try to send mails using C# I was receiving an error saying "The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.". But i was using the proper user name and password and cross checked those things many times.
Later I found that Zoho is using different mail server settings for Indian Data Center and United Sates Data Center. So If your account is setup in Indian Data Center use smtp.zoho.in while for US data centers use smtp.zoho.com
After login in to your email on browser check the URL to see which Data center you are using.