Certificate expired for Dodo POP server in Apple Mail

Just before Xmas 2025 I've been unable to download emails sent to my pop Dodo server on my Apple devices. I've been in contact with Apple, who told me to talk to Dodo. Dodo told me that Apple had changed its Mail settings and they couldn't do anything about it. Also that other pop servers had the same problem. I can read my mail on Dodo webmail if I have internet access and occasional emails come through, sometimes 2 days late, onto my iphone, Ipad & Macmini. I can still send emails from my dodo email server. Is this an Apple Mail problem and will it be solved?

The message I get from Mail is that the certificate for the server is invalid. When I ask for details I get a message 'Issued by E6' saying the Dodo certificate expired on 28 December. When I try to click that I trust the certificate, I get a message from Mail asking me to enter my password & update settings. But which password? I've tried all known passwords but still can't get in to update my Certificate Trust settings.

[Re-Titled By Moderator]