If the email is being sent from Marketo, you can edit the "From" and "Reply-to" addresses from the email editor itself.
If the email is being sent through Sales Insight, the email will have the "From" and "Reply-to" address set to the value of the Salesforce User Email Address field by default. You can change this setting in the Admin > Sales Insight tab to be a different email address field that is present on the Salesforce user. It will work if your reps change their Salesforce User Email Address but there may be a brief period where a sync will need to occur before the change has propogated. After they change their email address, make sure to send a test to ensure the change was processed.