This is probably an unpopular opinion but I do have some shared history with the OP.
Plusnet also registered a free domain for me way back then - it followed a data breach and I have been using the address ever since. I'm still with Plusnet and still have that address. However, I have had problems in the past with plusnet's email offering. It took a long time to get secure email implemented and there were quite a few issues over the years.
A few years back I decided that I'd use an outlook email address as my primary one and switched over to it for registrations and for writing to people. (I have a gmail address too courtesy of my android phone). Outlook is free, reliable and secure. Some might not like that the big tech companies are harvesting your data, but frankly, they do that everywhere and it's hard for the average person to avoid that. The Gmail mail address is set up to forward everything to Outlook and the plusnet domain is collected in Outlook via pop rather than imap so there is nothing left on PN's email server. That must be 4-5 years ago.
Mail still shows up addressed to the plusnet account. Most of it is spam and junk but very occasionally there is still some important stuff. My advice is to start to move away from PN for email but because there is no imminent decision for PN to withdraw email, you have time on your side and don't need to migrate it all in a rush. Just recognise it might be a while before your email identity switches from PN to something else more reliable.
I'm hoping for FTTP one day -I live in rural Norfolk where the infrastructure is terrible. I suspect that I might need a different ISP when that happens. I wouldn't want to have to rapidly migrate my email identity if / when that happens.
It worked for me.
