Not using a VPN.
The problem is triggered by' sending an email to c.25 recipients - not an unreasonable thing to do (a volunteer group email list - and I have larger lists which I've used without issue), When this happens, all emails - even a single recipient - are blocked for some hours. It started happening about a fortnight ago. I send roughly 6-10 email per day on average, usually to single recipients.
Re. the suggestion of spam - as a matter of course I went through due diligence to the best of my ability to check for this - multiple full scans against each device I used, checking network traffic volume, and running system restores to well before the point this started to happen.
As other users are suggesting, it looks like Plusnet have introduced some change that's had unintended consequences.
