If it's a new Plusnet account ( which I suspect it is from your post count of 1! ) then your account may not quite be configured completely yet for SMTP authentication. For the time being try unchecking the 'Require sign-in' box on the smtp server setup screen. It will work fine in this mode when you are connected via wifi to your PlusNet router ( you're connected to the Plusnet network ) but not when connected to another network e.g a public wifi.
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
