They've had email issues for years now
I've been with Plusnet since the early noughties but since around 2012, I noticed that emails took a while to arrive in my Inbox (didn't matter whether I was using their WebMail or my favourite email client), which was especially noticeable with password-reset emails, registration confirmation emails and the like, which would take at least 15 minutes to show up in my inbox. I often used my GMail account to register with websites and I'd get the confirmation email immediately, so it was obviously Plusnet's email wasn't that great. I've always used Thunderbird email client connecting to PN's IMAP server and occasionally TB would report connection issues, but it still retrieved my emails so despite that I was still happy, but when I recently started using another client (eM Client) because of it's superior filtering function making it much quicker and easier to cleanup my PN email, I found that it couldn't download all of my email from PN's IMAP server, a look at the logs showed that the IMAP server was randomly dropping the connection causing eM Client to stop what it was doing and throw it's hands up. I remembered trying to use Outlook 2019 with Plusnet a few months previously and it couldn't see any IMAP folders for my PN email except for Inbox, which I dismissed as Outlook being a POS and left it at that. A look at the eM Client forums and Plusnet Community forums revealed that many others were experiencing issues with the IMAP server connection being unexpectedly terminated, with forum posts going back to January 2021 and still the issue hadn't been fixed.
I decided to check out a some other paid email services (I didn't want to become the product of a free email provider) and settled on FastMail because it integrates with 1Password (which I've used for years now) to provide on-the-fly masked email addresses (makes it simple to identify any websites that have sold your email address) and provides loads of other useful features, plus it lives up to it's name and "just works".
Migrating my email from Plusnet was a doddle thanks to FastMail's import facility, though it took hours to complete because of the spurious dropped IMAP connections, but when it eventually completed, I was presented with a detailed log confirming that every email and folder had been imported. Just to be sure I eyeballed the contents of my PN IMAP folders with my FM IMAP folders just to make sure everything was there.
So with my email "out-sourced", there's now less to keep me from migrating to another ISP, though I'm happy with my Plusnet broadband connection. Sure, email is now costing me money but it's not too expensive and I found a 25% discount link. Plus a pay upfront 1 year, 2 year or 3 year subscription saves money.
If you're a Plusnet customer affected by their email issues, I'd recommend moving on to another email service. If you're not affected, you may well be sometime soon.