Once the email leaves our server we have no control over it. If it’s showing as “SENT”, as far as we can tell, it’s been successful.
It’s worth checking the client details too to see if there have been any bounces since the email was sent (see here for info: Email Complaints and Bounces). It’s worth noting however, if you use your own SMTP server, we’re not able to track them in this way.
If everything looks good, your client’s mail server may be discarding them without any warning.