I'll quickly explain how email tracking works.
When you send an email from Recruit CRM, a small "invisible" image of 1px is attached to your email. The person receiving your email never sees the image.
For email open tracking to work, your recipient's email client should allow images to be displayed. For some email clients, it on by default but for some email clients like Zoho, you might get options like the one in the image below.
Once your recipient clicks on display now, this particular email will be tracked and you will get a notification about your email being opened on Recruit CRM.
If your recipient doesn't click on display image, your email won't be tracked & you will not receive any notification on Recruit CRM.
Hope this helped :)