If someone is not getting email messages from Planning Center, then it's possible that it has been marked as spam or junk mail and is not being allowed through. The individual sending the message through PCO may get a return email like the following:

These are the steps to take to address this issue.
If possible contact the individual via another means (personal email, phone) and ask if they purposely blocked messages coming from GENESIS through Planning Center, and let them know that this blocks all messages and restricts our ability to communicate with them.
If they have purposely blocked our messages, and they no longer want to receive any email from us, we will need to remove them from the various mailing lists.
If they did not purposely block or they do want to receive communication from us, perform the following.
Whitelist the following email addresses:
After these steps you can follow up with the individual to make sure that they are now receiving messages sent from Planning Center.