The two actions that the Marketing Operations team should recommend to solve this issue are to delete any Leads or Contacts from Marketo Engage in the sync with no email address or invalid ‘-’ email address and to block unwanted Leads and Contacts from the sync based on a set criteria using the Custom - Sync Filter functionality. This is because these actions will help limit database growth and remove certain records from Marketo Engage by eliminating records that are not valid or useful for marketing purposes and preventing records that do not meet certain criteria from being synced. The other options are not as effective as these two, because they either rely on Dynamics functionality that may not be available or consistent, or they do not address the issue of database limits.

References: https://docs.marketo.com/display/public/DOCS/Microsoft+Dynamics+Sync%3A+Overview https://docs.marketo.com/display/public/DOCS/Custom+Sync+Filter