I recently began offering some freebies to people who open my email newsletters, and have had more than a few people who are subscribed say that they cannot find the email anywhere in their email program. Not in Spam, not in Trash. Nowhere.

In each case, Zoho Campaigns says that the email was delivered.

Can anyone help me understand how someone could have the email delivered to them, yet still be unable to find it? Providers include Gmail among others.