Not wishing to be contentious Spitoon, but that's like saying that fitting a new carburettor cured a blocked jet problem - as the installation of the new product will have carried out the full procedure in my para 2 above, it will have cured the problem.
"can't get it to send at the moment" "The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was..." That sounds quite normal, in that the email address was rejected. That means it got as far as the server so it was sent. Try sending to yourself, yourself at work, your neighbour etc, and see what happens.
Unfortunately these security problems are never easy to resolve - the error messages for example, are never clear. This results in users buying new bits and pieces everywhere. Find yourself a good PC support person and get them to review your PC - you'll be quids in as a result.