If you get this message on your client or journey invitation. This usually means you have sent the invite email to your client via the Adviser Portal itself and the system was unable to deliver the email successfully to them.
This is known as a bounce, this could be for a number of reasons and depending on the email provider of the client, then this could take up to 24 hours to happen or sometimes straight away.
If your invitation has bounced, then you will need to double check the email address is correct. If it is, then it could be that your clients inbox is full or in some circumstances that the client has put on their "Out of office" message on.