Updating salesforce when an email hard bounces

Best way is to create a smart campaign to send an alert email to the lead owner that this lead has hard bounced.

Here's what you could do:

1. In Salesforce create a new field for bounces. You could use the unsubscribe field if you dont want to create a new field, but that could potentially cause some confusion down the road.
2. In Marketo, create an alert email and a smart campaign.
   1. Use the trigger Email Bounces
   2. In the flow step choose to Change Data Value of the field you created to whatever you want the rep to see (or update unsubscribe to True) AND choose to Send Alert to the Lead Owner.

If you need to highlight this in SFDC on the lead/contact record, then you will need to create some custom fields in SFDC to pass data to them from Marketo.

Could have a check box for Hard Bounced, and a date field for when this happended.

How to do this:
Create smart campaign in Marketo, in smart list section, is email address bounced is any, the flow will be to mark new check box equals true and use a change data value flow step, select your new custom data field and use a system date token {{system.dateTime}}.

Here is a short video on this topic: https://community.marketo.com/MarketoResource?id=kA650000000GtoQCAS