I have not received an Email which was just sent.

There are several reasons due to which this may occur:

The MX record of the sender is misconfiguration.  As such, he should contact the technical support responsible for hosting his mailbox or correct the record him/herself

1.   Make sure, that the senders address has not been blacklisted by a global anti-spam organization.

2.   You may have exceeded your e-mail quota (if there is one for your e-mail account). The quota can easily be reviewed from your cPanel > Email Accounts – it should be represented by a bar, located next to each address on your hosting account.

If you may need further assistance on that matter, submit a Technical Support ticket from your Client Area so that we could resolve the issue on your behalf.