Mail error message: Connections to host p36-imap.mail.me.com on the default ports failed.

My wife's iCloud Mail account has stopped working on our shared iMac, showing the triangular warning sign and the error message "Connections to host p36-imap.mail.me.com on the default ports failed."

My own iCloud Mail account is working OK on the same iMac and she can receive and send her emails on her iPad and iPhone.

Is the problem with our iMac or Apple's server? And what can I do to fix it?

Thanks for any help.