Mail for Mac will not accept my password
You need to set up an app-specific password, which is now mandatory on anything connecting to iCloud that doesn't support 2FA. See Apple's current instructions here, but they may change over time and so shouldn't be included in an answer.
If the link no longer works, search for "icloud app-specific password".  

You will need to set up two-step (not two-factor, which your iMac doesn't support) authentication on your iCloud account.