We always recommend having a look at your spam folder before following the next steps!
You may receive notice that the email sent to your client has not been received. One of the reasons could be that the email is queued for delivery. This means that the clients email provider has accepted and stored our e-mail, however it’s not yet visible for the end-client.
In order to ensure that emails are being processed in a timely manner, you can add firstname.lastname@example.org as a Safe Sender.