Hello, how can i resolved my problem ? i using SMTP zoho having error 501 domain address need ELHO whats the means ?

220 mx.zohomail.com SMTP Server ready August 19, 2019 12:31:57 AM PDT 

hello: 501 Domain address required: EHLO

The following SMTP error was encountered: 501 Domain address required: EHLO 
Unable to send email using PHP SMTP. Your server might not be configured to send mail using this method.