Tracking emails from a different email account

A colleague of mine sent this email:

"We use a secondary email account for some of our communication with Accounts and Contacts - xxxxx@pmi.org.

We cannot track emails that have been previously sent, i.e., are in the 'Sent Items' into CRM.  I get the Track feature in the header of a new email from that email account.  Why can’t I retroactively track a Sent email from there like I can from my email account"?

I assume this is because that secondary email account is not synched with the CRM organization.  Any ideas on how to track those items?

Thanks.