I have been getting this error when trying to send out emails with my NUS account since 2am on Wednesday:
I called NUS IT Care and they say their engineers are working on it. They suggested a workaround which works: insert the domain name in the smtp settings. E.g. for Microsoft Entourage, this means going to Entourage > Account Settings… > Accounts > Edit Account (you can double-click the relevant account). to see this window.
Change receiving mail settings by adding the domain name to the userid like so:
- for staff – “nusstf\userid”
- for students – “nusstu\userid”.
My smtp settings will adopt these changes. This is because in the advanced sending options for “Sending Mail,” I have selected “Use same settings as receiving mail server”.
That worked for me.
There were other internet problems, like inability to use gmail and google docs, some said MSN was inaccessible. These seems to have since been fixed. NUS ITcare should have a twitter account, would be useful.
Otterman speaks... 
Thank you very much for your clear instructions! I had the exact same problem and I was able to solve it very quickly using your guide. Thank you again.
That’s really good to hear, Adam! Thanks for dropping a note.