Hi Ian,
You'll want to use one of our system fields for this purpose. When a Send Email flow step executes it checks four fields to see if a lead qualifies to receive an email:
Unsubscribed - used for when a lead requests not to receive Marketing Emails, will still receive operational
Marketing Suspended - For when you opt not to send non-operational emails to a lead
Email Invalid - Set when a bounce messaqge indicates that email address is invalid, will not be sent any email
Black Listed - Manually set for records that you do not want to receive any email, including operational
Depending on whether you want to send operational emails to these addresses you should use either Marketing Suspended, or Black Listed to manage these.