problem solved with'out calling the provider
For authentication:
username @ server.com
needs to be
username + server.com
Desknow can parse the @, apparently postfix/sasl can not. I also tried \\@ and so on, before +. So that's it for this issue.
Hope it helps others with the same configuration. Maybe putting it in the wiki would be a good thing.
Thanks to uxbod, ArcaneMagus for the prompt respons.
Regards