Document Approval - Email SMTP Unable to connect

Hi everybody

We are setting up NAV2013 in our company, and while configuring the document approval functions, we have a problem with sending the email.

the error message reads:

Fehler vom SMTP Mailsystem. Failure sending mail. Unable to connect to the remote server.

I believe we have the correct SMTP credentials with the port 25 and auth. set on anonymous, but this error message still appears on the NAV client of the person sending i.e. a purchase invoice for approval.

Do you have any idea how to resolve that?

Thanks