We're reaching out to let you know that we were unable to deliver some message(s) to your email

[#1]

We're reaching out to let you know that we were unable to deliver some message(s) to your
  email address due to security restrictions  on your mail server.

  It appears that your system settings is currently blocking incoming messages.

On top of not being able to access 2 of my accounts via Outlook pop3, i am now getting this message from clients on my main account that was working!

This is all turning into a bit of a shambles, and no one from Plusnet seems to have any anwsers yet 😞

Regards

Stuart

[#2]

PN is blacklisted on several mail servers; try changing your SMTP client to OUTLOOK;
https://www.lifewire.com/outlook-pop-server-settings-1170670

I don't use PN as ISP any more; I use M/S Exchange as my mail server, which when you set
up Outlook, and opt for POP3 is the default.

[#3]

That advice will only serve to cause more / different issues.  SPF directs that only Plusnet SMTP servers are authorised to send on behalf of Plusnet (et al) addresses.  Changing to another SMTP service will break SPF validation - at best it will then be treated as spam at worst its delivery will be blocked.