Student could have set preferences not to get email.
Messages isn't EMail so what you are looking at really doesn't show EMail.
Check your servers logs ... if on Linux, postfix logs are probably called maillog or mail.log and found where your operating system (not mentioned by you) keeps logs.
I assume that the postfix on your server is relaying to an SMTP server and email for this one student hasn't been shown to be relayed from the SMTP server.
If you have linux and command line and can install software, suggest installing 'alpine'. That's a text based MTA and will use your postfix for outgoing mail. Compose a test email message to the student. Then check your mail servers logs on the moodle ... then check smtp servers logs.
'SoS', Ken