Marketo Email templates that have been used are showing or appearing within the Salesforce Email Template Picker/Chooser.
The templates are created when Marketo syncs lead email activities to Salesforce. You can disable these activities from syncing in Marketo Admin > Salesforce > Edit Sync Options. Unselect the Email activities or Sales Email activities you have synced. The templates will no longer be created, but you also won't get these activities pushed from Marketo anymore. This applies to both Sales Insight and normal Marketo sent emails.
If you would like to note an email was sent to a lead without enabling these activities, you can utilize Interesting Moment Flow Steps or Create a Task to sync via the activity log!
Who This Solution Applies To
Customer using Sales Insight for Salesforce