Could you help me with this mail server disconnection problem?

I'm using your mail service on my wordpress website, with my domain email. 

Before, when people register on my website or forget the password, there are email notices sent out automatically. 

Now, it can't connect with zoho mail server,my website can't send register emails to new comer through connecting with smtp.zoho.com.   

Note: My email can login at mail.zoho.com,  receive & send emails as usual. 

Regards

Angelie