Did your SMTP settings change. Check to see if they are correct in Tools>account setting when you get to the settings page go to outgoing server check to see if this information is correct check with your ISP (Internet Service Provider)

all looks good ,but still no luck ,although yesterday i noticed that sometimes it was working fine and other times i got the message

bigpond suffer rather badly from incompetence and a disconnect between their technical people and the idiots on support.

If it works sometime and not others you can be almost certain that the good people at Bigpond have managed to mess it up again, unfortunately the IQ of their tech support people is so low that a conversation is about impossible, let along an actual useful solution. Last time I wasted time with them they blamed my copy of Microsoft Outlook and regardless of my assurances that it was simply not installed, refused to change their tack, or put me onto someone with a sufficient grasp on reality to at least consider blaming installed software.

My solution, and that of many others, is to simply not use their email accounts for communication with anyone but bogpond. If I had a choice, I would not be their customer at all, but where I live real choice is not something we have.