If the guest does not receive the emails, you must :
Check the validity of the email address, see if the word 'Error' appears under the column 'Last msg' for that particular guest.
Clicking on “Error” opens a page with the information regarding the error. For example, if the error comes from the domain name (the mention after the @), the following message will be displayed :
Check whether the word 'Opt-out' is displayed under the 'Last msg' column.