Incredimail is widely despised around here. (It makes loads of registry changes to PCs, and intensely annoys many recipients of email!)
If your friend insists upon using Incredimail, she should go to Tools > Accounts.
Click on the name of her email account and then on 'Properties'
Click the 'Servers' tab and check that the SMTP server is set to show
smtpmail.t-online.de
(Also check, using the 'Settings' and 'General' tabs', that the other settings are correct, in accordance with these instructions:
http://hilfe.telekom....de/3378/faq-155779971
Click 'Apply' and 'OK'
Chris