If you need the use PlusNet's webmail then you may be in for a problem as it has taken its webmail service offline indefinitely.

The problem is that it's mail software provided by @Mail has unpatchable flaws and will have to be replaced after security auditors investigated following a string of email problems at the ISP.
PlusNet may not be the only ISP affected as @Mail is common software package used by several ISPs.

A PlusNet update has been posted here.