It turns out that the solution is simple: in the account set-up, you need to ensure that 'Outgoing server requires authentication' is activated, and in 'Outgoing server settings' you need to select 'Use different user name for outgoing server' and re-enter the exact same details as for the incoming server (making sure you don't included the '@mac.com' on your account name).
kirkmc adds: I haven't tested this.

