We know for a fact that the current plusnet email system DOES do checks on the "Source address" when sending email. This is done as an anti-spam measure.
It appears to be driven by externally purchased "address lists" (quite normal).
One way a spam originator will send emails is via a VPN (to make it difficult to track who is sending the spam!), thus these lists will often block email sent via a VPN (technical note: when you send an email from home, it will originate from your broadband IP, when you send it over a VPN, it will originate from one of the VPN provider's IP's. It's therefore easy for the email provider to see who is trying to connect!)
Blocking connections from VPNs generally is quite common: one of the the main reasons people will use a VPN is to pretend to be in the UK, when they are actually in, say, China. As a web site, maybe I only wish to deal with ACTUAL UK users, who live in the UK (I don't wish to export my product)... so I block users accessing via known VPNs (I used to provide training for a product that included this ability)
There's 2 solutions:
1) don't use the VPN to send email;
2) move your email service to someone who DOES allow connections via a VPN.
(And yes, I know the latter is a PITA, I have a personal email address - so won't need to change it - and moving THAT has enough issues. For most users, they are still using an "@plus.net" email address (or similar), so that's ALSO gonna need a change of email address. But you'd need to do that anyway, if you changed your broadband supplier!)
