After update from Outlook 2010 to 2020, I can't add extra accounts.

[Teeafit]

Using Outlook 2010 for years, I had a general POP3 account <businessname@domain.co.uk> (with domain being a Fasthosts one... just using Outlook to handle the emails). I also had extra accounts such as <personal@domain.co.uk> — these would send with their own signature and 'Reply to' address. The incoming mails to those addresses were forwarders to <businessname@domain.co.uk>, so that everything came in to the same general Inbox, but had their separate identities. Everything was saved locally to outlook.pst.

I have now updated to Outlook 2020, and set up the general account to IMAP, transferring emails to outlook.pst as appropriate. But try as I might, I can't set up the extra identities. I click the 'Add Account' button and select POP3, but whatever I do (whether I let Outlook set it up for itself, or do it manually) it is always 'Something went wrong — we couldn't connect to the incoming POP3 server'. So I'm unable to send from <personal@domain.co.uk>... this was a very useful facility that I want to continue with.

I've tried alternative incoming and outgoing servers, but the message is always the same. I'm obviously missing something.