Email Sending delay in wordpress website

Email Sending delay in wordpress website

Dear Concern,

I am using Zoho mail plugin for one of my wordpress website. Zoho handles all the email delivery of the website. I have integrated Eventespresso with my website. Whenever a user registers in the website, he gets a notification via email. Sometimes it takes 4-5 minutes to receive the email and sometimes it takes 1-2 hours to receive the email. Would you please tell me where to look for the problem?

Regards,
Hasib.