When you change or delete a field in Salesforce.com, your marketing assets or campaigns may need changes. Marketo automatically emails the account admins when this occurs.
It's up to you to correct any references to those changed fields in your assets; you may need to make copy changes, remove form fields, or update emails to remove those references. You can use the Field Management tool to find where a field is being used:
If you've deleted a field from Salesforce and no longer want it in Marketo, you can easily hide them from the user interface as long as they're no longer in use. (Marketo doesn't allow you to delete fields from the application.) Instructions for hiding a field are in the Field Management documentation:
If you change a multi-value field or select box's values in Salesforce, Marketo does not update those values in your form fields. You need to check if the form labels or values need to be updated to match the changes in Salesforce.com. If you do make changes to a form, remember to re-approve the landing pages that use the form