If you see the term Generation Rejection in your failed emails list it indicates the following;
We have tried to send the email to the owner/contact's email address but their email server has sent it back with a message that the user - or their automated system- has marked the email as spam and we should not send any more emails to this address.
If we continue to send emails to this address their server may blacklist our mail server which would stop all our emails from all users being sent to that server.
As the integrity of our email server is imperative that email address is added to a suppression list which stops any more email being sent until the issue has been fixed.
How to stop peoples email servers from automatically blocking the emails?
Please send this link and ask you client to follow the steps on found here
This will inform their mail server that they wish to receive these email and NOT to mark them as spam.
Once the user has done this please get them to send an email to firstname.lastname@example.org and we can remove them from the suppression list.