If your email has been working fine up until recently and you haven't made any changes to your email client or network settings, you may be experiencing general internet connectivity issues. Check to see if you can access websites using a web browser.
If you are able to visit websites, check whether you're only experiencing problems sending emails, receiving emails, or both.
If you use an SMTP (outgoing) server other than your incoming server (POP or IMAP) the problem is most likely with the SMTP server of your Internet Service Provider, e.g. Bigpond. They could be experiencing a temporary outage with their mail servers. Just wait a while, say 20 minutes, before contacting their Helpdesk.