Ask them to check their junk/spam folder, in case it ended up in there. If it did, ask them to ‘whitelist’ or add the email as a contact so it doesn’t happen again.
If it’s not in the junk box, it may be that their email provider has blocked the email with its firewall. This can happen as we have to ‘mask’ your email addresses/domains in order to send emails that look to have come from you. There is little that can be done about this.
We suggest that any ‘bulk’ emailing is done via Campaign Monitor to maximise the chances of it being received, plus it allows you to see if the email has been opened/clicked/forwarded etc.