Did you recently change internet service providers? The "not on my list of allowed..." message usually appears when you've got an email address with one provider but are using another provider to connect to the internet.
For example, if you use an eircom.net email address, but connect to the internet with bt broadband, you have to send emails via bt, not via eircom.net
If this is the case, you will have to re-set your outgoing mail server in the mail settings in outlook.
(I use outlook express, so don't have the exact instructions for outlook, sorry)