Error when sending out email via SMTP

Error when sending out email via SMTP

Hi, I recently started encountering an error when using my Zohomail account to send out emails. The error message was : 

sendEmail.exe[16528]: ERROR => ERROR => SMTP-AUTH: Authentication to smtppro.zoho.com:587 failed. rmtree failed: Directory not empty 

I have been using this without any issue for the past few years but this error started only this month. Can anybody help with what this error means?