iMAC and iPhone Mail App - 'can't connect to the account'

Hello and Help!

I've had no problems until yesterday. Suddenly I can receive email but not send out on my iMAC or iPhone. All software is up to date.

It states that it "can't connect to the account" and asks for the password.

The password is then NOT accepted.

HOWEVER, if I go to iCloud on the web and sign in using the password (Apple ID) I can get access to the email and send and receive from there.

Does anyone have any ideas please?

Thank you!

Dee