On send email from Contact, add activity to custom object if bcc email matches custom object's email

Question

I have a custom object named 'Host'. When they are BCC'd in an email, I want to log that in the Host's Activity History.

I'm a bit lost on where to get started on this. It seems as if there was a trigger that was 'after Email' where I could somehow add the email to the Host's Activity History would be the most ideal - but I don't think this exists.

Any suggestions?

EDIT:

More Info:

When you go to the 'Contact' Objects in salesforce, and select a contact, you are able to send them an email. You do this by hovering over the 'Active History' button and selecting 'Send Email'.

In the form that follows, there is a section marked BCC where you can send a copy of the email to other email addresses. If an email specified in the BCC field matches an Email attached to my custom 'Host' object, I want to log the email in that 'Host' Active History

Answer 1

@cropredy was right, I needed to use a trigger on EmailMessage. Unfortunately, Salesforce doesn't allow you to tie the email directly to a Custom Object(https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_emailmessagerelation.htm)

Thus, I had to create a Completed task on my custom host and log the details of the email in the comments.

It's not ideal, but it works