I can receive emails on my imac but cannot sent them. Am prompted for my password and when I give it, it says it is incorrect.

I am able to receive emails on my new imac, but am unable to send emails. They languish in my outbox and are never sent. I am then prompted to give my password. When I do, am told that it isn't valid. Very frustrating, as I am new to apple and have never been prompted to give passwords and prove who I was with my PC.