Why do some, but not all of my POP3 emails come in to CRM?

Why do some, but not all of my POP3 emails come in to CRM?

I've just configured my Zoho CRM Email settings to pull emails for my leads & contacts from my Office 365 POP3 account. I have dozens of messages on my Outlook with corresponding email addresses in CRM, but only a handful have been pulled into my CRM Mail. 

Why are all emails not showing in CRM?

Thanks - Jeff