It is normal to receive mailings that were undeliverable. Most of the time, this is due to vacant housing units. The normal range of what we typically see if 3-10% of the postcards mailed are returned because they are undeliverable.
Our recommendations for mailing sample size take the estimated percentage of undeliverable addresses into account. Sending out a new batch of survey invitations would add to the cost of the project and extend the timeline. For these reasons, we recommend against sending out another wave of mailings for those returned postcards, except in the cases where the undeliverable rate is exceptionally high.