How delete an imap.gmail.com certificate which does not show up in Keychain?

Ok the saga with my mum's Mini and her emails continues.

I wanted to change her Gmail to IMAP but she didn't like that and wanted to revert to POP. Fine, I said, and changed everything back but now Mail says the certificate is invalid:

The certificate for this server is invalid. You might be connecting to server that is pretending to be "imap.gmail.com" which could put your confidential information at risk. Would you like to connect to the server anyway?

If I click "View Certificate", I get the error that the "certificate is invalid (host names do not correspond)". Well of course, because it says imap.gmail.com and I am trying to reach pop.gmail.com.

This is really odd because both Gmail and Mail.app are correctly set up to use POP. There's no IMAP account left on the computer afaik because I've deleted it from Sys Prefs. And - even more weirdly - the above certificate does not show up in Keychain, even when I search for imap or gmail.

Would really appreciate a bit of speedy help because I'll travel tomorrow so I'd like to sort this out soonish.

Help! TIA
Philip

 

Great, thanks Bear. That did the trick. No need to delete anything, now it works. Many thanks!