Email marked as delivered, but receipient claims they didn't recieve it

Recently we sent out an email, and checked in on some of the recipients. For some of them, it marked as delivered, but it wasn’t in their inbox/spam. What explanations are there for this, and is there anyway to prevent that from happening in the future?

Thank you.

Hi @leah1,

HubSpot has dedicated troubleshooting steps for this topic here: Understand why contacts didn't receive marketing emails

Best regards!