Hi @JamesB
Are you using the default email setup, or have you changed it to your own SMTP setup?
If it’s the default, once the email has left our servers it’s difficult to understand why it doesn’t arrive, but it’s most likely to be down to the spam/junk filters on your clients end, filtering it out without any warning.
If a message is returned to us, you will see that the email has bounced from the client record. There’s more on this here.