Sender email address (domain) not changing?

I must be missing something really daft here.
My user object has an override set for the internet domain name.
The PO has no override
The Domain has no override
Yet when I send an email, it's going out with the default domain name as specified at system level.

If the override option doesn't override that setting, how do I send an email out using a different domain name in the sender address?

I'm certain we used to do this just by setting the individual user object to have a different domain name, but it's not working at all now?

Thanks Alan