How to get Interworx authenticate when sending email?

[pingu]

I have Interworx set up with nodeworx users email to info@mydom.se - this emailaccount is on a windows-webbhosting server (same subnet as Interworx).
Problem is when Interworx sends an email to peterh@mydom.se it can't be delivered:Connected to 190.18.19.20 but sender was rejected.
Remote host said: 551 This mail server requires authentication before sending mail from a locally hosted domain. Please reconfigure your mail client to authenticate before sending mail.
So Interworx can't send to peterh@mydom.se but to info@mydom.se??
I set up backup-jobs with email-notification to myself, that's when I get this strange error.

[IWorx-Paul]

Hi pingu,

Is the mydom.se mail server 190.18.19.20? That seems like a message that would happen if the 190.18.19.20 mail server did not handle mail for the mydom.se domain, and thought you were trying to relay.

Paul

[pingu]

No, it is the mail server for mydom.se, no relaying.
And how come Interworx can send to info@mydom.se but not to peterh@mydom.se?