unable to verify usename or password

Good afternoon everyone

I am trying to add an email account to my iMac and MacBook and the message I received is 'unable to verify account name or password'. This is very frustrating as I have used the same settings to successfully add this email account to my iPhone.

I have seen this question posed a number of times but no-one seems to have the answer.

Can anyone help please ?

Thanks, Mikey.