Getting Error 550 when trying to setup plusnet email in outlook 2007

[#1]

Just joined PlusNet and trying to setup email in Outlook 2007. I've set as explained in all the plusnet guide sheets for setup the email but keep getting the 550 error when trying the Test button.
Webmail works fine. Contacted PlusNet support and they can't help saying their end is fine and setting in outlook seem fine.
As anybody else had the same problem

[#2]

Have a look here http://community.plus.net/forum/index.php/topic,144892.0.html if you are running Windows 10.

[#3]

Thanks, I'm running 8.1

[#4]

Error 550 is a generic error. Is any other text given which might help diagnose the specific problem?